tag:theconversation.com,2011:/ca/topics/gamma-ray-bursts-2295/articlesGamma ray bursts – The Conversation2023-06-07T18:36:02Ztag:theconversation.com,2011:article/2071332023-06-07T18:36:02Z2023-06-07T18:36:02ZBrightest cosmic explosion of all time: how we may have solved the mystery of its puzzling persistence<figure><img src="https://images.theconversation.com/files/530418/original/file-20230606-17-8pwe5c.jpg?ixlib=rb-1.1.0&rect=54%2C38%2C2101%2C2117&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">An x-ray of the brightest ever gamma ray burst reflected off dust layers, creating extended 'light echoes' of the initial blast. </span> <span class="attribution"><span class="source">Nasa</span></span></figcaption></figure><p>First <a href="https://www.darpa.mil/about-us/timeline/vela">detected accidentally</a> by US military satellites in the late 1960s, cosmic explosions known as gamma ray bursts (GRBs) have come to be understood as the brightest explosions in the universe.</p>
<p>Typically, they <a href="https://www.space.com/gamma-ray-burst.html">are the result</a> of the cataclysmic birth of a black hole in a distant galaxy. One way this can happen is through the collapse of a single, massive star.</p>
<p>Astronomers such as myself working in the field are well aware of the massive energy scales involved in GRBs. We know they can release as much energy in gamma rays as the Sun does throughout its lifetime. But every once in a while, an event is observed that still gives us pause.</p>
<p>In October 2022, gamma-ray detectors on the orbital satellites Fermi and the Neil Gehrels Swift Observatory <a href="https://science.nasa.gov/grb-221009a">noted a burst</a> known as GRB 221009A (the date of detection). </p>
<p>This quickly turned out to be a record-setter. It was dubbed the Brightest Of All Time, or the “Boat”, as convenient shorthand among astronomers studying and observing the event. Not only did the Boat start out bright, it refused to fade away like other bursts.</p>
<p>We still do not fully know why the burst was so exceptionally bright, but our new study, <a href="http://www.science.org/doi/10.1126/sciadv.adi1405">published in Science Advances</a>, provides an answer for its stubborn persistence. </p>
<p>The burst originated from a distance of 2.4 billion light years – relatively nearby for a GRB. But even when accounting for relative distance, the energy of the event and the radiation produced by its aftermath were off the charts. It is decidedly not normal for a cosmically distant event to deposit about a gigawatt of power into the Earth’s upper atmosphere.</p>
<h2>Observing narrow cosmic jets of gas</h2>
<p>GRBs such as the Boat launch a stream of gas moving at very close to light speed into space. How exactly the jet is launched remains something of a puzzle – but most likely, it involves magnetic fields near where the black hole is being formed. </p>
<p>It is the early emission from this jet that we see as the burst. Later, the jet slows down and produces additional radiation, a fading afterglow of light – from radio waves up to (in exceptional cases) gamma rays.</p>
<p>We do not observe jets directly. Instead, like distant stars, we see GRBs as points in the sky. Nevertheless, we have good reason to believe that GRBs do not explode in all directions equally. For GRB 221009A, this would certainly be unreasonable, as it would involve multiplying the amount of energy detected on Earth by all other directions – amounting to much more energy than any star would have available.</p>
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<p>Another indication that GRBs come from jets pointing roughly at us is due to special relativity theory. Relativity teaches us that the speed of light is constant, no matter how fast a source moves at us. But that still allows for the <em>direction</em> of light to become distorted. Thanks to this fun-house mirror effect, light emitted in all directions from the surface of a fast-moving jet will end up focused strongly along its direction of motion. </p>
<p>That said, the edges of a jet heading in our direction will be very slightly curved away, meaning their light is focused away from our direction. Only later, when the jet slows down, do the edges normally come into view and does the afterglow start to fade faster.</p>
<p>But here again, GRB 221009A broke the rules. Its edges never showed, and it joined a select group of very bright bursts that refuse to fade normally. Rather than starting to fade slowly and then disappearing quickly, it is steadily fading over time. </p>
<p>In our work, we demonstrate how the appearance of the jet edges can be obscured in a way that matches the observations of the Boat. The key idea is as follows: yes, a narrow jet was launched, but it had a difficult time escaping the collapsing star, leading to a lot of mixing with stellar gas along the sides of the jet.</p>
<h2>From simulation to observation</h2>
<p>To test whether this was indeed the case, we took <a href="https://academic.oup.com/mnras/article/500/3/3511/5974546">a computer simulation result</a> showing this mixing and implemented it in a model that could actually be compared to the Boat data directly. And it showed that what would normally be a quick turnover to a strongly fading signal, now became a drawn-out affair. </p>
<p>Radiation from the dying star’s shock-heated gas kept appearing in our line of sight, explaining why it stayed so bright. This kept happening all the way up to the point that any characteristic jet signature was lost in the overall emission. </p>
<p>This way, GRB 221009A not only confirms expectations from simulation, but also provides a clue to similarly bright events seen in the past, where people had to keep <a href="https://academic.oup.com/mnras/article/462/1/1111/2589937">revising the energy estimate upwards</a> while waiting for a jet edge to show. </p>
<p>We calculated that the likelihood of seeing a burst this bright is about one in a thousand years, so we are lucky to have spotted one. But questions remain. What role do magnetic fields play, for example?</p>
<p>Theorists and numerical modellers will be exploring these matters for years, scouring the Boat data while we stay on the lookout for the next big event to arrive</p><img src="https://counter.theconversation.com/content/207133/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Hendrik Van Eerten receives funding from the UK's Science and Technology Facilities Council (STFC) and from the European Research Council (ERC).</span></em></p>Radiation from the brightest cosmic explosion ever seen may have been mixing with gas and dust around its dying star – making the signal last longer.Hendrik Van Eerten, Reader in Astrophysics, University of BathLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1747262022-01-12T21:28:51Z2022-01-12T21:28:51ZBlack holes: we think we’ve spotted the mysterious birth of one<figure><img src="https://images.theconversation.com/files/440422/original/file-20220112-19-1yretgy.jpeg?ixlib=rb-1.1.0&rect=0%2C0%2C4992%2C2979&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Image of a Wolf Rayet star – potentially before collapsing into a black hole.</span> <span class="attribution"><a class="source" href="https://www.eso.org/public/images/eso2115b/">ESO/L. Calçada</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Astronomers are increasingly drawing back the curtains on black holes. In the past few years, we <a href="https://theconversation.com/first-black-hole-photo-confirms-einsteins-theory-of-relativity-115167">have finally captured actual photos</a> of these fearsome creatures and measured the gravitational waves – ripples in spacetime – <a href="https://theconversation.com/gravitational-waves-found-the-inside-story-54589">that they create when colliding</a>. But there’s still a lot we don’t know about black holes. One of the biggest enigmas is exactly how they form in the first place. </p>
<p>My colleagues and I now believe we have observed this process, providing some of the best indications yet of exactly what happens when a black hole forms. Our results are published in two papers in <a href="https://www.nature.com/articles/s41586-021-04155-1">Nature</a> and the <a href="https://arxiv.org/abs/2111.12110">Astrophysical Journal</a>.</p>
<p>Astronomers believe, on both observational and theoretical grounds, that most black holes form when the centre of a massive star collapses at the end of its life. The star’s core normally provides pressure, or support, using heat from intense nuclear reactions. But once such a star’s fuel is exhausted and nuclear reactions stop, the inner layers of the star collapse inward under gravity, crushing down to extraordinary densities. </p>
<figure class="align-center ">
<img alt="Image of a black hole." src="https://images.theconversation.com/files/440259/original/file-20220111-16-71qdkv.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/440259/original/file-20220111-16-71qdkv.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=350&fit=crop&dpr=1 600w, https://images.theconversation.com/files/440259/original/file-20220111-16-71qdkv.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=350&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/440259/original/file-20220111-16-71qdkv.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=350&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/440259/original/file-20220111-16-71qdkv.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=439&fit=crop&dpr=1 754w, https://images.theconversation.com/files/440259/original/file-20220111-16-71qdkv.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=439&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/440259/original/file-20220111-16-71qdkv.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=439&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">First image of a black hole.</span>
<span class="attribution"><span class="source">Event Horizon Telescope collaboration et al.</span></span>
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</figure>
<p>Most of the time, this catastrophic collapse is halted when the star’s core condenses into a solid sphere of matter, rich in particles called neutrons. This leads to a powerful rebound explosion that destroys the star (<a href="https://spaceplace.nasa.gov/supernova/en/">a supernova</a>), and leaves behind an exotic object known as a neutron star. But <a href="https://phys.org/news/2021-09-heavier-stars-supernovae-quietly-implode.html">models of dying stars show</a> that if the original star is massive enough (40-50 times the mass of the Sun), the collapse will simply continue unabated until the star is crushed down into a gravitational singularity – a black hole.</p>
<h2>Explosive theories</h2>
<p>While stars collapsing to form neutron stars are now routinely observed throughout the universe (supernova surveys find dozens of new ones every night), astronomers are not yet entirely sure what happens during the collapse to a black hole. Some pessimistic models suggest the entire star <a href="https://www.pnas.org/content/117/3/1240">would be swallowed up without much of a trace</a>. Others propose that the collapse to a black hole would produce <a href="https://academic.oup.com/mnrasl/article/485/1/L83/5420382">some other kind of explosion</a>. </p>
<p>For example, if the star is rotating at the time of collapse, some of the infalling material may be focused into jets that escape the star at high velocity. While these jets wouldn’t contain much mass, they’d pack a big punch: if they slammed into something, the effects might be quite dramatic in terms of the energy released.</p>
<p>Up until now, the best candidate for an explosion from the birth of a black hole has been the strange phenomenon known as long-duration <a href="https://theconversation.com/a-collapsing-star-in-a-distant-galaxy-fired-out-some-of-the-most-energetic-gamma-rays-ever-seen-127114">gamma-ray bursts</a>. First discovered in the 1960s by military satellites, these events have been hypothesised to result from jets accelerated to mindboggling speeds by newly formed black holes in collapsing stars. However, a longstanding problem with this scenario is that gamma-ray bursts also expel abundant radioactive debris that continues to shine for months. This suggests most of the star exploded outward into space (as in an ordinary supernova), instead of collapsing inward to a black hole.</p>
<p>While this doesn’t mean a black hole can’t have been formed in such an explosion, some have concluded that other models provide a more natural explanation for gamma ray bursts than a black hole forming. For example, a <a href="https://academic.oup.com/mnras/article/413/3/2031/967037">super-magnetised neutron star</a> could form in such an explosion and produce powerful jets of its own.</p>
<h2>Mystery solved?</h2>
<p>My colleagues and I, however, recently uncovered a new and (in our view) much better candidate event for creating a black hole. On two separate occasions in the past three years – once in 2019 and once in 2021 – we witnessed an exceptionally fast and fleeting type of explosion that, much like in gamma-ray bursts, originated from a small amount of very fast-moving material slamming into gas in its immediate environment.</p>
<p>By using spectroscopy – a technique that breaks down light into different wavelengths – we could infer the composition of the star that exploded for each of these events. We discovered that the spectrum was very similar to so-called “<a href="https://theconversation.com/your-smiles-cosmic-history-we-discovered-the-origin-of-fluoride-in-early-galaxies-169562">Wolf-Rayet stars</a>” – a very massive and highly-evolved type of star, named after the two astronomers, Charles Wolf and Georges Rayet, that first detected them. Excitingly, we were even able to rule out a “normal” supernova explosion. As soon as the collision between the fast material and its environment ceased, the source practically vanished – rather than glowing for a long time.</p>
<p>This is exactly what you would expect if, during the collapse of its core, the star ejected only a small amount of material with the rest of the object collapsing downward into an enormous black hole.</p>
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<a href="https://images.theconversation.com/files/440428/original/file-20220112-17-1bq6djd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Artist's image of the explosion. compared to a supernova and a gamma ray burst." src="https://images.theconversation.com/files/440428/original/file-20220112-17-1bq6djd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/440428/original/file-20220112-17-1bq6djd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=273&fit=crop&dpr=1 600w, https://images.theconversation.com/files/440428/original/file-20220112-17-1bq6djd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=273&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/440428/original/file-20220112-17-1bq6djd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=273&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/440428/original/file-20220112-17-1bq6djd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=343&fit=crop&dpr=1 754w, https://images.theconversation.com/files/440428/original/file-20220112-17-1bq6djd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=343&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/440428/original/file-20220112-17-1bq6djd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=343&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">The new study observed two events that may belong to third type of explosion, lasting only a short time.</span>
<span class="attribution"><span class="source">Credit: Bill Saxton, NRAO/AUI/NSF</span></span>
</figcaption>
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<p>While this is our favoured interpretation, it’s not the only possibility. The most prosaic one is that it was a normal supernova explosion, but that a vast shell of dust formed in the collision, concealing the radioactive debris from view. It’s also possible that the explosion is of a new and unfamiliar type, originating from a star we’re not familiar with. </p>
<p>To answer these questions, we will need to search for more such objects. Until now these kinds of explosions have been difficult to study because they are fleeting and hard to find. We had to use several observatories together in quick succession to characterise these explosions: the <a href="https://www.ztf.caltech.edu/">Zwicky Transient Facility</a> to discover them, the <a href="https://www.schoolsobservatory.org/learn/eng/tels/lt">Liverpool Telescope</a> and the <a href="https://www.chetec-infra.eu/tna/tna-tels/au-not/">Nordic Optical Telescope</a> to confirm their nature, and large high-resolution observatories (the <a href="https://www.nasa.gov/mission_pages/hubble/main/index.html">Hubble Space Telescope</a>, Gemini Observatory, and the <a href="https://www.eso.org/public/unitedkingdom/teles-instr/paranal-observatory/vlt/">Very Large Telescope</a>) to analyse their composition.</p>
<p>While we didn’t initially know exactly what we were seeing when we first discovered these events, we now have a clear hypothesis: the birth of a black hole.</p>
<p>More data from similar events may soon be able to help us verify or falsify this hypothesis and establish the link to <a href="https://theconversation.com/the-cow-explosion-how-astronomers-are-cracking-one-of-the-greatest-new-mysteries-of-the-sky-109590">other types of unusual, fast explosions</a> that our team and others have been finding. Either way, it seems this truly is the decade we crack the mysteries of black holes.</p><img src="https://counter.theconversation.com/content/174726/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Daniel Perley does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>It’s long been a mystery how black holes form, now astronomers are on the verge of cracking it.Daniel Perley, Reader of Astrophysics, Liverpool John Moores UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1591712021-10-05T07:58:51Z2021-10-05T07:58:51ZEnergy burst from most distant known galaxy might have been a satellite orbiting Earth<figure><img src="https://images.theconversation.com/files/404007/original/file-20210602-15-848qx3.jpeg?ixlib=rb-1.1.0&rect=241%2C181%2C5277%2C3078&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.nasa.gov/feature/goddard/2019/hubble-studies-gamma-ray-burst-with-highest-energy-ever-seen">NASA, ESA and M. Kornmesser</a></span></figcaption></figure><p>The cosmos is the stage for a variety of giant explosions. These include stellar flares, where stars suddenly release magnetic energy; and neutron star mergers, where two dense stars collide together. But one class of explosions outshines the rest: gamma ray bursts are the most energetic explosions seen in the universe. </p>
<p>Gamma rays are one of the most energetic forms of light, and gamma ray bursts release almost unimaginable quantities of them. First discovered during the cold war – by military satellites searching for the signs of nuclear tests in the upper atmosphere – <a href="https://theconversation.com/how-we-created-a-mini-gamma-ray-burst-in-the-lab-for-the-first-time-89933">gamma ray bursts</a> are now thought to be caused by massive stars undergoing huge explosions when they run out of fuel. These events are rare, but so energetic they can be seen in galaxies many billions of light years away. </p>
<p>Recently, <a href="https://www.nature.com/articles/s41550-020-01266-z">astronomers thought</a> they had seen evidence for one of these explosions from the most distant galaxy every seen. But a <a href="https://www.nature.com/articles/s41550-021-01472-3">recently published paper</a> casts doubt on these claims, suggesting it might have been caused by a more mundane source much closer to home.</p>
<h2>Gamma ray bursts</h2>
<p>No gamma ray bursts have been documented in our galaxy yet, which may not be a bad thing. A gamma ray burst pointed directly at the Earth would probably lead to a mass extinction event, and the end of civilisation as we know it. Undocumented events may in fact already have caused mass extinction events <a href="https://www.nature.com/articles/news030922-7">in Earth’s history</a>. </p>
<p>Gamma ray bursts have been seen far away, however. <a href="https://www.nature.com/articles/s41550-020-01266-z">The paper</a> suggesting researchers had discovered a new gamma ray burst in the most distant known galaxy was published in 2020. Using the <a href="https://www.keckobservatory.org">Keck telescope</a> on Mauna Kea, Hawaii, the researchers observed strips of the sky, and happened to see a bright flash, just a few seconds long, in one of their exposures. </p>
<p>By modelling the duration and brightness of the flash, they ruled out the possibility that it was a natural or human-made satellite close to home. They also ruled out a number of other astronomical explanations, and concluded that the most likely explanation was, indeed, a gamma ray burst. </p>
<p>What was so unique about this discovery was that the team pinpointed the direction of the event and found it was coming from the same area as a galaxy known as <a href="https://www.nasa.gov/feature/goddard/2016/hubble-team-breaks-cosmic-distance-record">GN-z11</a>, which just so happens to be the most distant and oldest galaxy we’ve yet discovered. </p>
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<p>Was this an incredible cosmic coincidence? Or was this a sign that gamma ray bursts were more common in the very early universe, just 400 million years after the big bang? The latter conclusion would have big implications for our understanding of how stars and galaxies form in the early universe, and led to a lot of excitement among astronomers.</p>
<p>But unease about the conclusions of the group surfaced, with some arguing it was much more likely that the flash was from an object within our solar system, which could be a natural (such as a moon) or artificial satellite. In <a href="https://arxiv.org/abs/2101.12738">another paper</a>, a different team suggested the most likely explanation was a reflection from a human-made satellite. The original authors <a href="https://arxiv.org/abs/2102.01239">followed up</a> on these claims, doubling down on their gamma ray burst interpretation, but the chorus of doubters was only getting louder.</p>
<h2>Space junk</h2>
<p>Now, the controversy has taken another turn, with a <a href="https://www.nature.com/articles/s41550-021-01472-3">new paper</a> recently published in Nature Astronomy. The authors of this paper suggest the purported gamma-ray burst was in fact a flash caused by a human-made satellite after all. The researchers used a public <a href="https://www.space-track.org/">space-track website</a> to search for possible human satellite interference in the direction and at the time of the flash detection. </p>
<figure class="align-center ">
<img alt="telescope pointed at starry sky, and inset image of distant galaxy" src="https://images.theconversation.com/files/424491/original/file-20211004-27-1ubgq4p.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/424491/original/file-20211004-27-1ubgq4p.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=414&fit=crop&dpr=1 600w, https://images.theconversation.com/files/424491/original/file-20211004-27-1ubgq4p.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=414&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/424491/original/file-20211004-27-1ubgq4p.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=414&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/424491/original/file-20211004-27-1ubgq4p.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=520&fit=crop&dpr=1 754w, https://images.theconversation.com/files/424491/original/file-20211004-27-1ubgq4p.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=520&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/424491/original/file-20211004-27-1ubgq4p.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=520&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The Roman Baranowski Telescope/Poznań Spectroscopic Telescope used to confirm the man-made nature of the flash, and the position of GN-z11.</span>
<span class="attribution"><span class="source">Krzysztof Kamiński, Adam Mickiewicz University in Poznań</span></span>
</figcaption>
</figure>
<p>Around the time that the original team were studying the sky, a Russian <a href="https://www.space.com/40397-proton-rocket.html">proton rocket</a> reached low Earth orbit and released its upper stages (dubbed Breeze-M), which then became space junk, orbiting the Earth. By looking at the orbit of <a href="https://theconversation.com/space-debris-what-can-we-do-with-unwanted-satellites-40736">the space debris</a> and matching with the observations taken in the original study, the new team found the flash could be simply explained by the upper stage falling past the part of the sky the telescope was observing. </p>
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Read more:
<a href="https://theconversation.com/the-suns-atmosphere-is-hundreds-of-times-hotter-than-its-surface-heres-why-161392">The Sun's atmosphere is hundreds of times hotter than its surface – here's why</a>
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<p>The proton rocket has been in operation since the 1960s, and it’s not the only time one of its Breeze-M upper stages has been in the news. In 2013 an explosion scattered <a href="https://spacenews.com/nasa-breeze-m-debris-a-threat-to-satellites-not-iss/">huge amounts of debris</a> into near Earth orbit, and left NASA scrambling to assess whether it would pose a danger to the International Space Station.</p>
<p>While this particular incident was perhaps particularly unlucky, with <a href="https://theconversation.com/thousands-more-satellites-will-soon-orbit-earth-we-need-better-rules-to-prevent-space-crashes-154014">increasing amounts</a> of junk in space, and the launching of large <a href="https://theconversation.com/theres-a-parking-crisis-in-space-and-you-should-be-worried-about-it-83479">constellations of satellites</a> by the private company SpaceX and others in the coming years, it highlights the increasing difficulties astronomers face observing from the Earth’s surface. </p>
<p>Better databases of satellites and space debris will help avoid these kinds of misidentifications. But the increasing light pollution from satellite constellations threatens the ability of telescopes on the ground to even see clearly enough to do world-leading science.</p>
<p><em>This article has been corrected as it initially said the recent paper was published in Nature. The correct paper is Nature Astronomy.</em></p><img src="https://counter.theconversation.com/content/159171/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Christopher Lovell does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Did we observe the most distant gamma ray burst yet seen, or was it something closer to home?Christopher Lovell, Postdoctoral Researcher in Astronomy, University of HertfordshireLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1271142019-11-20T19:16:19Z2019-11-20T19:16:19ZA collapsing star in a distant galaxy fired out some of the most energetic gamma rays ever seen<figure><img src="https://images.theconversation.com/files/302593/original/file-20191120-515-11nq98w.jpg?ixlib=rb-1.1.0&rect=925%2C609%2C3488%2C1953&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The HESS telescopes in Namibia are on the alert for high-energy gamma rays.</span> <span class="attribution"><span class="source">HESS Collaboration / Clementina Medina</span></span></figcaption></figure><p>The brightest fireworks in the universe are called gamma-ray bursts and are created by the death throes of certain kinds of stars. These intense blasts release as much energy in one second as the Sun will over its whole lifetime, but we still don’t understand exactly how they do it.</p>
<p>We are getting closer, however. We recently had the best ever look at the incredibly high-energy gamma rays emitted by these bursts. Gamma rays are like particles of visible light, but each of these high-energy rays carries as much as 100 billion times more energy. </p>
<p>New <a href="https://www.nature.com/articles/s41586-019-1743-9">research</a> published this week in Nature by <a href="https://www.nature.com/articles/s41586-019-1750-x">two</a> teams of scientists from around the world (I am a member of one of them) reveals the gamma rays are more energetic than we knew and that the afterglow of the burst lasts much longer.</p>
<h2>What are gamma-ray bursts?</h2>
<p>In the late 1960s, secret spy satellites designed to look for gamma-ray flashes from nuclear explosions began to detect mysterious bursts of gamma rays coming from outer space. It was not until the 1990s that scientists began to unravel the mystery with data from new satellites. </p>
<p>We now believe at least some of these gamma-ray bursts are caused by the collapse of super-massive stars. Many stars end their lives in an enormous explosion called a supernova, but very heavy stars can create an even bigger blast called a hypernova. </p>
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Read more:
<a href="https://theconversation.com/flash-aah-aah-could-a-gamma-ray-burst-eradicate-all-life-on-earth-5291">Flash, aah-aah! Could a gamma ray burst eradicate all life on Earth?</a>
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<p>In this process, the star’s core collapses and becomes a rapidly rotating black hole. The surrounding gas forms a spinning disk around the black hole, which then creates a narrow, intense jet of radiation. If this jet is pointing towards Earth, we can see it as a bright gamma-ray burst, which typically lasts no more than a minute or two. </p>
<p>The very high-energy gamma rays are given off by matter that is accelerated to very close to the speed of light as it whirls around the black hole.</p>
<p>Because the bursts are rare and don’t last long, it can be difficult to get a good look at one with a telescope.</p>
<h2>The afterglow</h2>
<p>On July 20 2018, gamma-ray and X-ray satellites alerted the world to a new gamma-ray burst, named GRB 180720B. It was a very strong burst and lasted for about 50 seconds – a relatively long duration, indicating the death of a massive star. </p>
<p>Following the alert, several observatories around the world immediately began observing the spot in the sky that the burst came from. About 10 hours later, that spot came into view for the High-Energy Stereoscopic System (HESS) gamma-ray telescopes in Namibia.</p>
<p>Even though 10 hours had passed, HESS was able to <a href="https://www.nature.com/articles/s41586-019-1743-9">observe the afterglow</a> of the burst, which still included extremely energetic gamma rays.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/302350/original/file-20191119-12539-yfiqm6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/302350/original/file-20191119-12539-yfiqm6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/302350/original/file-20191119-12539-yfiqm6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=568&fit=crop&dpr=1 600w, https://images.theconversation.com/files/302350/original/file-20191119-12539-yfiqm6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=568&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/302350/original/file-20191119-12539-yfiqm6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=568&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/302350/original/file-20191119-12539-yfiqm6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=714&fit=crop&dpr=1 754w, https://images.theconversation.com/files/302350/original/file-20191119-12539-yfiqm6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=714&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/302350/original/file-20191119-12539-yfiqm6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=714&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Very high-energy gamma rays from the gamma-ray burst GRB 180720B, 10 to 12 hours after the burst, as seen by the large HESS telescope.</span>
<span class="attribution"><span class="source">Supplied</span>, <span class="license">Author provided</span></span>
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</figure>
<p>At HESS we have been looking at other gamma-ray bursts in this way for more than a decade. This was the first time it detected gamma rays from the high-energy afterglow at energies never seen before.</p>
<p>While we had anticipated the detection of gamma-ray bursts at these high energies, the discovery that they were still around many hours after the initial burst came as a great surprise. </p>
<p>This result suggests the accelerated particles creating the gamma rays still exist or are created a long time after the explosion, which is hard to explain with our existing theories of what happens in gamma-ray bursts.</p>
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<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/an-extragalactic-mystery-where-do-high-energy-cosmic-rays-come-from-6623">An extragalactic mystery: where do high-energy cosmic rays come from?</a>
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<h2>Even more energy</h2>
<p>Also just published are observations of a different gamma-ray burst (GRB 190114C)
made using the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes in the Canary Islands. </p>
<p>The MAGIC astronomers caught the “prompt” early stages of the burst, detecting gamma rays with even more energy – some of the <a href="https://www.nature.com/articles/s41586-019-1750-x">most energetic ever seen</a>.</p>
<p>These detections show we still have much to learn about gamma-ray bursts. But they also give us confidence that our methods to detect them are improving. We will be able to study plenty more in the future with the much more sensitive <a href="https://www.cta-observatory.org">Cherenkov Telescope Array</a>, which is now under construction.</p><img src="https://counter.theconversation.com/content/127114/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Gavin Rowell receives funding from The University of Adelaide for his involvements in the High Energy Stereoscope System (HESS) Collaboration.</span></em></p>Mysterious cosmic flashes known as gamma-ray bursts are caused by the death throes of massive stars.Gavin Rowell, Associate Professor in High Energy Astrophyics, University of AdelaideLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1095902019-01-10T22:15:39Z2019-01-10T22:15:39Z‘The Cow’ explosion: how astronomers are cracking one of the greatest new mysteries of the sky<figure><img src="https://images.theconversation.com/files/253203/original/file-20190110-32151-17ggi9s.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The cow erupted near a galaxy known as CGCG 137-068, marked by the yellow cross.</span> <span class="attribution"><span class="source">Credit: Sloan Digital Sky Survey</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Something highly unusual was picked up by the <a href="https://www.fallingstar.com/home.php">Asteroid Terrestrial Impact Last Alert System</a> (ATLAS) on June 16, 2018. The discovery of a strange flare in the sky – brightening and fading – was reported in an <a href="http://www.astronomerstelegram.org/?read=11727">astronomical telegram</a>, which then alerted scientists. The source was named AT2018cow – based on a naming convention for transient sources where the last three letters are <a href="https://wis-tns.weizmann.ac.il/search?name=2018cow">randomly assigned</a>. Understandably, though, scientists quickly nicknamed it “the Cow”.</p>
<p>Astronomers all around the world started <a href="https://earthsky.org/space/mystery-explosion-at2018cow-nicknamed-the-cow">watching the Cow</a>, which is one of the strangest and most observed objects recently discovered. Measurements of its brightness and wavelength suggest that it is located close to 200m light years away, far outside our own galaxy. This means that the explosion would have been extremely bright to have been detected at all, producing as much energy as the sun radiates in 1,700 years.</p>
<p>Today, a race is on to work out exactly what it is. Our new research, to be <a href="https://arxiv.org/abs/1808.08492">published in the Monthly Notices of the Royal Astronomical Society</a>, has come up with one explanation.</p>
<p>The Cow must have appeared extremely suddenly – it was not seen in a search just 32 hours earlier. Follow up observations using instruments in space and on the ground suggested its brightness was larger than expected from a supernova (an exploding star). A gamma ray burst – an intense explosion of light – would have been another possibility, but the accompanying X-ray emission from the event seemed too small for it to fit this description. </p>
<p>The temperature profile of the source indicated that the object was glowing at a temperature of about 26,000°C, with peak brightness in the ultraviolet region of light. Assuming the brightness of such a thermal source was a sphere, the size came out to be very large – about 50 times the orbit of the Earth around the sun. A supernova would also extend to something similar in size.</p>
<p>A second surprise was that the spectrum – a measurement of how light breaks down according to wavelength – did not show the same features as that of a typical supernova. It was like the glow of a hot body only. Some faint but broad bumps and dips suggested there was, however, some material emitting at extremely high velocity in a cocoon around the hot glowing body.</p>
<h2>Swift rescue</h2>
<p>As a member of the <a href="https://swift.gsfc.nasa.gov/">Neil Gehrels Swift observatory</a> (in short “Swift”) team, I was asked to dig into our data to find out more. Swift is a satellite in a low-Earth orbit which is able to point to a new source automatically in about 90 seconds after a gamma ray burst, or after being commanded to do so from the ground. Swift has many telescopes and can provide data in a range of different light regions – from the visible and ultraviolet to gamma rays. </p>
<p>At the time, several other teams were investigating the source using various observatories. Papers based on the <a href="https://arxiv.org/abs/1807.06369">Swift’s X-ray data</a> and the <a href="https://arxiv.org/abs/1807.05965">ATLAS data</a> had already been published. Dan Perley from Liverpool John Moores University and his collaboration was <a href="https://arxiv.org/abs/1808.00969">just finishing</a> the Herculean task of analysing the optical and infrared data from the large <a href="https://www.growth.caltech.edu">GROWTH collaboration</a> – comprising telescopes around the world – following up on interesting sources. </p>
<p>By the last weeks of August, all this research had made it clear that the Cow did definitely not fit the profile of a normal gamma ray burst, nor a supernova or a superluminous supernova (an extremely bright supernova). This left the possibility, however, that what we were seeing was a star being ripped apart by a black hole – a so called tidal disruption. </p>
<p>This possibility became a focus of our investigation. With the multi-wavelength Swift data in hand, and with the already published preprints providing additional insight in certain wavelengths of light, we had the opportunity to narrow the search.</p>
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<p>We decided to combine well understood physics with the observations like using the distance at which a black hole swallows a star whole, the distance at which a black hole rips a star apart, and on that basis wrote our paper. We suggest that a white dwarf (a star that has reached the end of its life) of 0.1 or 0.4 times the mass of the sun being ripped apart by a black hole of 100,000 to a million solar masses would be able to explain what we see. In fact, this would match the observations measured in several regions of light – from gamma rays to radio waves. Such an event would also provide a natural means to form the jet of material that was observed. </p>
<h2>Other candidates</h2>
<p>But there are other possibilities. Two recent papers led by <a href="https://arxiv.org/abs/1810.10720">Anna Ho from Caltech</a> and <a href="https://arxiv.org/abs/1810.10720">Rafaella Margutti from Northwestern University</a> respectively suggest that the Cow was an “engine-driven” explosion in which a fast-spinning neutron star (a very dense star) that formed in an exotic form of supernova was pumping energy into the expanding material. This research also leaves open the possibility that it is a black hole doing the pumping.</p>
<p>Sadly, we missed the early stages of the event which may have contained clues as to which process can actually generate such enormous amounts of energy. Hopefully, with more and better telescopes guarding the sky, we can catch similar events at earlier times going forward. </p>
<p>An object like this may also be a source of gravitational waves – ripples in spacetime. That means observatories like the LISA satellite which the European Space Agency is building now <a href="https://theconversation.com/lisa-pathfinder-will-pave-the-way-for-us-to-see-black-holes-for-the-first-time-51374">may be able to actually see them</a>. Holy cow, that would be amazing.</p><img src="https://counter.theconversation.com/content/109590/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Paul M. Kuin does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Odd event could be explained by a star being ripped apart by a black hole.Paul M. Kuin, Senior Research Scientist, UCLLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/899332018-01-15T11:32:52Z2018-01-15T11:32:52ZHow we created a mini ‘gamma ray burst’ in the lab for the first time<figure><img src="https://images.theconversation.com/files/201791/original/file-20180112-101511-10lbvxe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Illustration of a gamma ray burst in space.</span> <span class="attribution"><span class="source">ESO/A. Roquette</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Gamma ray bursts, intense explosions of light, are the <a href="https://www.space.com/23684-brightest-gamma-ray-burst-mysteries.html">brightest events ever</a> observed in the universe – lasting no longer than seconds or minutes. Some are so luminous that they can be observed with the naked eye, such as the burst “GRB 080319B” discovered by <a href="https://swift.gsfc.nasa.gov/">NASA’s Swift GRB Explorer</a> mission on March 19, 2008.</p>
<p>But despite the fact that they are so intense, scientists don’t really know what causes gamma ray bursts. There are even people who believe some of them might be <a href="https://www.haystack.mit.edu/hay/staff/jball/revgrbs.ps">messages sent from advanced alien civilisations</a>. Now we have for the first time managed to recreate a mini version of a gamma ray burst in the laboratory – opening up a whole new way to investigate their properties. Our research <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.185002">is published</a> in Physical Review Letters.</p>
<p>One idea for <a href="http://science.sciencemag.org/content/323/5922/1688">the origin of gamma ray bursts</a> is that they are somehow emitted during the emission of jets of particles released by massive astrophysical objects, such as black holes. This makes gamma ray bursts extremely interesting to astrophysicists – their detailed study can unveil some key properties of the black holes they originate from.</p>
<p>The beams released by the black holes would be mostly composed of electrons and their “antimatter” companions, the positrons – all particle have antimatter counterparts that are exactly identical to themselves, only with opposite charge. These beams must have strong, self-generated magnetic fields. The rotation of these particles around the fields give off powerful bursts of gamma ray radiation. Or, at least, this is what our <a href="https://academic.oup.com/mnras/article/369/1/197/1052906">theories predict</a>. But we don’t actually know how the fields would be generated.</p>
<p>Unfortunately, there are a couple of problems in studying these bursts. Not only do they last for short periods of time but, most problematically, they are originated in distant galaxies, sometimes even billion light years from Earth (imagine a one followed by 25 zeroes – this is basically what one billion light years is in metres).</p>
<p>That means you rely on looking at something unbelievably far away that happens at random, and lasts only for few seconds. It is a bit like understanding what a candle is made of, by only having glimpses of candles being lit up from time to time thousands of kilometres from you. </p>
<h2>World’s most powerful laser</h2>
<p>It has been recently proposed that the best way to work out how gamma ray bursts are produced would be by mimicking them in small-scale reproductions in the laboratory – reproducing a little source of these electron-positron beams and look at how they evolve when left on their own. Our group and our collaborators from the US, France, UK, and Sweden, recently succeeded in creating the first small-scale replica of this phenomenon by using one of the most intense lasers on Earth, the <a href="https://www.clf.stfc.ac.uk/Pages/Laser-system-Gemini.aspx">Gemini laser</a>, hosted by the Rutherford Appleton Laboratory in the UK. </p>
<p>How intense is the most intense laser on Earth? Take all the solar power that hits the whole Earth and squeeze it into a few microns (basically the thickness of a human hair) and you have got the intensity of a typical laser shot in Gemini. Shooting this laser onto a complex target, we were able to release ultra-fast and dense copies of these astrophysical jets and make ultra-fast movies of how they behave. The scaling down of these experiments is dramatic: take a real jet that extends even for thousands of light years and compress it down to a few millimetres. </p>
<p>In our experiment, we were able to observe, for the first time, some of the key phenomena that play a major role in the generation of gamma ray bursts, such as the self-generation of magnetic fields that lasted for a long time. These were able to confirm some major theoretical predictions of the strength and distribution of these fields. In short, our experiment independently confirms that the models currently used to understand gamma ray bursts are on the right track.</p>
<p>The experiment is not only important for studying gamma ray bursts. Matter made only of electrons and positrons is an extremely peculiar state of matter. Normal matter on Earth is predominantly made of atoms: a heavy positive nucleus surrounded by clouds of light and negative electrons. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/201915/original/file-20180115-101508-1kndf3p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/201915/original/file-20180115-101508-1kndf3p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=586&fit=crop&dpr=1 600w, https://images.theconversation.com/files/201915/original/file-20180115-101508-1kndf3p.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=586&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/201915/original/file-20180115-101508-1kndf3p.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=586&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/201915/original/file-20180115-101508-1kndf3p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=736&fit=crop&dpr=1 754w, https://images.theconversation.com/files/201915/original/file-20180115-101508-1kndf3p.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=736&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/201915/original/file-20180115-101508-1kndf3p.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=736&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Artist impression of gamma ray burst.</span>
<span class="attribution"><span class="source">NASA</span></span>
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<p>Due to the incredible difference in weight between these two components (the lightest nucleus weighs 1836 times the electron) almost all the phenomena we experience in our everyday life comes from the dynamics of electrons, which are much quicker in responding to any external input (light, other particles, magnetic fields, you name it) than nuclei. But in an electron-positron beam, both particles have exactly the same mass, meaning that this disparity in reaction times is completely obliterated. This brings to a quantity of fascinating consequences. For example, sound would not exist in an electron-positron world. </p>
<p>So far so good, but why should we care so much about events that are so distant? There are multiple reasons indeed. First, understanding how gamma ray bursts are formed will allow us to understand a lot more about black holes and thus open a big window on how our universe was born and how it will evolve.</p>
<p>But there is a more subtle reason. SETI – Search for Extra-Terrestrial Intelligence – <a href="https://theconversation.com/its-not-all-about-aliens-listening-project-may-unveil-other-secrets-of-the-universe-45031">looks for messages from alien civilisations</a> by trying to capture electromagnetic signals from space that cannot be explained naturally (it focuses mainly on radio waves, but gamma ray bursts are associated with such radiation too). </p>
<p>Of course, if you put your detector to look for emissions from space, you do get an awful lot of different signals. If you really want to isolate intelligent transmissions, you first need to make sure all the natural emissions are perfectly known so that they can excluded. Our study helps towards understanding black hole and pulsar emissions, so that, whenever we detect anything similar, we know that it is not coming from an alien civilisation.</p><img src="https://counter.theconversation.com/content/89933/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Gianluca Sarri receives funding from the Engineering and Physical Sciences Research Council.</span></em></p>New research may help us to look for messages from alien civilisations.Gianluca Sarri, Lecturer at the School of Mathematics and Physics, Queen's University BelfastLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/849572017-10-16T14:03:49Z2017-10-16T14:03:49ZWhy astrophysicists are over the moon about observing merging neutron stars<figure><img src="https://images.theconversation.com/files/190212/original/file-20171013-3555-ldwh37.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Simulation of two neutron stars merging.</span> <span class="attribution"><a class="source" href="https://svs.gsfc.nasa.gov/10740">NASA/AEI/ZIB/M. Koppitz and L. Rezzolla</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>When <a href="https://www.ligo.caltech.edu/">LIGO</a>, the Laser Interferometer Gravitational-Wave Observatory, first detected <a href="https://www.ligo.caltech.edu/page/what-are-gw">gravitational waves</a> from merging black holes, it opened up a new window in astrophysics and provided the most powerful confirmation yet of Einstein’s theory of general relativity. Now LIGO has done it again, together with the <a href="https://www.ego-gw.it/public/about/whatIs.aspx">Virgo interferometer</a>, this time by <a href="https://doi.org/10.1103/PhysRevLett.119.161101">observing merging neutron stars</a> – something astrophysicists had known must happen but had never been able to detect definitively until now.</p>
<p>Observing two neutron stars smash together is important for much more than just the thrill of discovery. This news may confirm a longstanding theory: that some gamma-ray bursts (GRBs for short), which are among the most energetic, luminous events in the universe, are the result of merging neutron stars. And it is in the crucible of these mergers that most heavy elements may be forged. Researchers can’t produce anything like the temperatures or pressures of neutron stars in a laboratory, so observation of these exotic objects provides a way to test what happens to matter at such extremes.</p>
<p>Astronomers are excited because for the first time they have gravitational waves and light signals stemming from the same event. These truly independent measurements are separate avenues that together add to the physical understanding of the neutron star merger.</p>
<h2>Gravitational waves just one part of this news</h2>
<p>The LIGO project has thus far announced the detection of four mergers of binary black holes – observed via the gravitational waves they emitted. These are ripples in the fabric of spacetime propagating in all directions, like waves emanating out from a pebble dropped in a pond. Encoded in the gravitational wave signal is information about the pre- and post-merger masses of the objects. Black holes are much more massive than neutron stars, so the energy they release as gravitational waves is much higher. Because light cannot escape from a black hole, you expect (and see) no light from these mergers.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/190240/original/file-20171014-3537-84dvk7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/190240/original/file-20171014-3537-84dvk7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/190240/original/file-20171014-3537-84dvk7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190240/original/file-20171014-3537-84dvk7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190240/original/file-20171014-3537-84dvk7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190240/original/file-20171014-3537-84dvk7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190240/original/file-20171014-3537-84dvk7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190240/original/file-20171014-3537-84dvk7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Artist’s rendering of a gamma-ray burst, the most energetic form of light.</span>
<span class="attribution"><a class="source" href="https://svs.gsfc.nasa.gov/12055">NASA/Swift/Cruz deWilde</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>The merger of neutron stars should produce both a gravitational wave and a short gamma-ray burst signal. These brief, incredibly intense flashes of gamma-ray light are seen from galaxies across the universe. They come in two types, classified by their duration. Short GRBs are thought to <a href="https://doi.org/10.1016/j.physrep.2007.02.005">come from the mergers of neutron stars</a>, while long GRBs are known to be coincident with supernovas.</p>
<p>Key to unlocking the mystery of any astronomical object is knowing its distance. In recent years, astronomers have <a href="https://doi.org/10.1086/498107">identified the host galaxies</a> of a <a href="https://doi.org/10.1086/512664">handful of short GRBs</a>. Determining those galaxies’ distances allows astronomers to calculate the power emitted in gamma-rays during the burst, and to determine (or rule out) physical scenarios that could produce that power.</p>
<p>But for LIGO to detect two <a href="https://doi.org/10.1103/PhysRevD.93.112004">neutron stars spiraling in toward each other and merging</a>, it would need to happen relatively nearby – within around 250 million light-years. That such an event was not detected during the first year and a half of LIGO observations already lets astronomers place a constraint on how frequently they happen in the nearby universe.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/190213/original/file-20171013-3537-t0lobc.gif?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/190213/original/file-20171013-3537-t0lobc.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/190213/original/file-20171013-3537-t0lobc.gif?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190213/original/file-20171013-3537-t0lobc.gif?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190213/original/file-20171013-3537-t0lobc.gif?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190213/original/file-20171013-3537-t0lobc.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190213/original/file-20171013-3537-t0lobc.gif?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190213/original/file-20171013-3537-t0lobc.gif?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Galaxy NGC 4993 seemed unassuming enough….</span>
<span class="attribution"><a class="source" href="http://stdatu.stsci.edu/dss/index.html">Palomar Observatory – Space Telescope Science Institute Digital Sky Survey</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>So the rumor of a merging neutron star detection by LIGO with a coincident short gamma-ray burst (<a href="https://gcn.gsfc.nasa.gov/other/170817A.gcn3">GRB170817A</a>) seen by NASA’s <a href="https://fermi.gsfc.nasa.gov/">Fermi Gamma-ray Space Telescope</a> spread through the astronomical community like wildfire this past summer. Astronomers watched from the sidelines as most of the major telescopes in (and above) the world slewed toward an otherwise unremarkable old, nearby (130 million light-years) elliptical galaxy named NGC 4993.</p>
<h2>What we’ve known about neutron stars</h2>
<p>Most stars end their lives relatively calmly; no longer supported by the fusion of hydrogen into helium, their outer layers glide slowly off into space while their cores collapse to the very limits allowed by normal matter – burning embers the size of the Earth called white dwarf stars.</p>
<p>For the rare stars whose masses are a bit higher, 10 to 20 times that of the sun, the picture is a bit different. These stars die the way they lived: quickly and violently, ejecting their outer layers as supernovas and leaving behind something far stranger – a neutron star.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/190251/original/file-20171015-3527-ykrc0b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/190251/original/file-20171015-3527-ykrc0b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/190251/original/file-20171015-3527-ykrc0b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=945&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190251/original/file-20171015-3527-ykrc0b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=945&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190251/original/file-20171015-3527-ykrc0b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=945&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190251/original/file-20171015-3527-ykrc0b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1187&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190251/original/file-20171015-3527-ykrc0b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1187&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190251/original/file-20171015-3527-ykrc0b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1187&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Nobel Prize-winning physicist Subrahmanyan Chandrasekhar.</span>
<span class="attribution"><a class="source" href="http://www.apimages.com/metadata/Index/AP-A-IL-CX3-OBIT-CHANDRASEKHAR/a3ada89cc6e0da11af9f0014c2589dfb/2/0">AP Photo</a></span>
</figcaption>
</figure>
<p>The details of this story were worked out in 1930 by then 19-year-old Indian astrophysicist <a href="http://chandra.harvard.edu/about/chandra.html">Subrahmanyan Chandrasekhar</a>. He determined precisely how far you can compress normal matter before the relentless pressure of gravity forces electrons into the nuclei of their atoms where they merge with protons to form neutrons. Instead of an Earth-sized remnant, a massive star’s core collapses further to become a highly compressed ball of exotic matter as small as a city but whose mass can be twice that of the sun.</p>
<p>Neutron stars rotate incredibly rapidly. The collapse from millions to tens of kilometers in extent increases their spin due to conservation of angular momentum, like an ice skater pulling in her arms. While the parent star may have rotated once a month, a newly born neutron star can spin hundreds of times per second.</p>
<p>This rapid spinning led to their initial discovery. 50 years ago, Antony Hewish and Jocelyn Bell Burnell <a href="https://www.atnf.csiro.au/outreach/education/everyone/pulsars/index.html">discovered the first radio pulsar</a>: a neutron star emitting radio waves which appear to observers as pulses as the star rotates, like a lighthouse. <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1974/">Hewish would win the 1974 Nobel Prize in physics</a> for this discovery, while Bell Burnell was controversially overlooked.</p>
<p>But what are neutron stars really made of? Are they neutrons all the way through or can they break down further again, into what physicists call “quark soup”? The answer lies in measuring their size. A larger neutron star is mostly neutrons, a smaller star has a more complicated interior made of quarks – the building blocks of protons and neutrons. Untangling how this works is important for our understanding of the fundamental properties of subatomic particles. <a href="https://www.nasa.gov/nicer">A new telescope on the International Space Station</a> aims to address this question by targeting neutron stars and measuring their sizes.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/190239/original/file-20171014-3524-1hqa2i9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/190239/original/file-20171014-3524-1hqa2i9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/190239/original/file-20171014-3524-1hqa2i9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=393&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190239/original/file-20171014-3524-1hqa2i9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=393&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190239/original/file-20171014-3524-1hqa2i9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=393&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190239/original/file-20171014-3524-1hqa2i9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=494&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190239/original/file-20171014-3524-1hqa2i9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=494&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190239/original/file-20171014-3524-1hqa2i9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=494&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The orbiting neutron stars rapidly lose energy by emitting gravitational waves and merge after about three orbits, or in less than 8 milliseconds. A black hole forms and the magnetic field becomes more organized, eventually producing structures capable of supporting the jets that power short gamma-ray bursts.</span>
<span class="attribution"><a class="source" href="https://svs.gsfc.nasa.gov/10740">NASA/AEI/ZIB/M. Koppitz and L. Rezzolla</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>When neutron stars merge</h2>
<p>Over half of all stars are part of binary pairs, and massive stars are more likely to occur in binaries. These pairs of massive stars will co-evolve, and when they die, a pair of neutron stars may remain, orbiting one another.</p>
<p>An orbiting pair of neutron stars loses energy by emitting gravitational waves, and over time this loss of energy will cause them to migrate closer and closer until they eventually collide. While the eventual merger is nearly instantaneous, the gradual inspiral takes tens to hundreds of millions of years, so we expect to see mergers in more evolved galaxies – like NGC 4993, for instance – rather than those that are still rapidly forming new stars.</p>
<p>For decades, it has been <a href="https://doi.org/10.1086/181612">suggested that merging neutron stars</a> may <a href="https://doi.org/10.3847/2041-8205/829/1/L13">provide a mechanism for producing most of the elements</a> on the periodic table heavier than iron. These so-called r-process elements must form in a neutron-rich environment, and have been formed by humans only during the explosion of nuclear bombs.</p>
<p>The signal from such an event is suspected to rapidly cascade through the electromagnetic spectrum, from gamma-rays to X-rays, visible light and infrared. Known as kilonovas, <a href="https://doi.org/10.1038/nature12505">these afterglows have been seen</a> from past short GRBs.</p>
<p>Finally all the pieces fall into place with this gravitational wave detected by the LIGO and Virgo teams, and all the subsequent supporting observations made by astronomers around the world. We know the neutron star masses, the duration of the event, and the distance of the host galaxy. This not only confirms the hypothesis that <a href="https://doi.org/10.3847/2041-8213/aa920c">merging neutron stars produce short GRBs</a>; it lays the foundation for astronomers to produce models of the merger backed both by fundamental physics and real world observations. It’s a rare event to see something new for the first time, and rarer still that it confirms a longstanding theory.</p><img src="https://counter.theconversation.com/content/84957/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Roy Kilgard has received funding from NASA through the Space Telescope Science Institute and from the Smithsonian Astrophysical Observatory.</span></em></p>The gravitational wave itself is the least exciting part of the announcement from LIGO and Virgo. Observing this new source answers many longstanding questions.Roy Kilgard, Research Associate Professor of Astronomy, Wesleyan UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/856472017-10-16T14:02:45Z2017-10-16T14:02:45ZHow we discovered gravitational waves from ‘neutron stars’ – and why it’s such a huge deal<figure><img src="https://images.theconversation.com/files/190387/original/file-20171016-31010-1rr1trx.jpg?ixlib=rb-1.1.0&rect=0%2C243%2C1710%2C1324&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist's illustration of two merging neutron stars.</span> <span class="attribution"><span class="source">National Science Foundation/LIGO/Sonoma State University/A. Simonnet.</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Rumours have been <a href="https://www.scientificamerican.com/article/rumors-swell-over-new-kind-of-gravitational-wave-sighting/">swirling for weeks</a> that scientists have detected <a href="https://theconversation.com/explainer-gravitational-waves-and-why-their-discovery-is-such-a-big-deal-53239">gravitational waves</a> – tiny ripples in space and time – from a source other than colliding black holes. Now we can finally confirm that we’ve observed such waves produced by the violent collision of two massive, ultra-dense stars more than 100m light years from the Earth. </p>
<p>The discovery was made on August 17 by the <a href="https://theconversation.com/gravitational-waves-discovered-how-did-the-experiment-at-ligo-actually-work-54510">global network of advanced gravitational-wave interferometers</a> – comprising the twin LIGO detectors in the US and their European cousin, Virgo, in Italy. It is hugely important, not least because it helps solve some big mysteries in astrophysics – including the cause of bright flashes of light known as “<a href="https://theconversation.com/flash-aah-aah-could-a-gamma-ray-burst-eradicate-all-life-on-earth-5291">gamma ray bursts</a>” and perhaps even the origins of heavy elements such as gold.</p>
<p>As a member of the LIGO scientific collaboration, I was immediately in raptures as soon as I saw the initial data. And the period that followed was definitely the most intense and sleep deprived, but also incredibly exciting, two months of my career. </p>
<p>The announcement comes just weeks after three scientists <a href="https://theconversation.com/scientists-behind-the-discovery-of-gravitational-waves-win-the-2017-nobel-prize-for-physics-66457">were awarded the Nobel Prize in Physics</a> for their foundational work leading to the discovery of gravitational waves, first announced in February 2016. Since then, detecting gravitational waves from colliding black holes has started to feel like familiar territory – <a href="https://theconversation.com/experiments-simultaneously-detect-gravitational-waves-and-help-open-up-a-new-era-of-astronomy-84818">with four further such events detected</a>. But as far as we know, colliding black holes offer purely a window on the dark side of the universe. We haven’t been able to register light from these events with any other instruments.</p>
<p>But GW170817 – the catchy title for the event of August 17 — changes all that. That’s because the source of the waves this time was two “<a href="https://www.nasa.gov/mission_pages/GLAST/science/neutron_stars.html">neutron stars</a>” – incredibly dense stellar remnants the size of a city, each weighing more than the sun. These stars whizzed around each other at a sizeable fraction of the speed of light before merging in a cataclysmic collision that we’ve now seen shake the very fabric of space and time.</p>
<h2>Mysteries solved</h2>
<p>The cosmic concerto was just beginning, however. Astronomers have long suspected that the merger of two neutron stars could be the overture to a short <a href="https://theconversation.com/flash-aah-aah-could-a-gamma-ray-burst-eradicate-all-life-on-earth-5291">gamma ray burst</a> – an intense flash of gamma-ray light that releases more energy in a fraction of a second than the sun will pump out in ten billion years. For several decades we have observed these gamma ray bursts, but without knowing for sure what causes them.</p>
<p>However, just 1.7 seconds after the gravitational waves from GW170817 arrived at the Earth, <a href="https://www.nasa.gov/content/fermi-gamma-ray-space-telescope/">NASA’s Fermi satellite</a> observed a short burst of gamma rays in the same general region of the sky. LIGO and Virgo had found the smoking gun, and the link between neutron star collisions and short gamma ray bursts was finally and clearly established.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Many hands make light (and gravity) work. NASA’s Fermi satellite was instrumental in the discovery.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>The combination of gravitational-wave and gamma-ray observations allowed the position of the cosmic explosion to be pinpointed to less than 30 square degrees on the sky – or about 100 times the size of the full moon. This, in turn, allowed a whole barrage of astronomical telescopes sensitive to light across the entire electromagnetic spectrum to search this small patch of sky for the aftermath of the explosion. And sure enough this was found – in an unfashionable backwater towards the edge of a fairly <a href="https://en.wikipedia.org/wiki/NGC_4993">unassuming galaxy called NGC4993</a>, in the constellation of Hydra. </p>
<p>Over the next few days and weeks astronomers watched agog as the embers from the explosion glowed brightly and faded, beautifully matching the pattern expected for <a href="http://theconversation.com/we-beat-a-cyber-attack-to-see-the-kilonova-glow-from-a-collapsing-pair-of-neutron-stars-85660">a so-called “kilonova”</a>. This is produced when material rich in subatomic particles known as neutrons from the initial merger is ejected at great speed by the gamma ray burst. This ploughs into the surrounding region of space, triggering the production of heavy radioactive elements. </p>
<p>These unstable elements typically split up (decay) to a stable state by emitting radiation. This is what causes the glow of the kilonova, which we have now confirmed by mapping it out in exquisite detail. Our observations also strongly support the theory that the stable end-products of these chains of reactions include copious amounts of precious metals like gold and platinum. While we’ve suspected neutron stars to be key to <a href="https://www.smithsonianmag.com/science-nature/all-the-gold-in-the-universe-could-come-from-the-collisions-of-neutron-stars-13474145/">producing these elements in space</a>, that hypothesis now looks a whole lot more convincing. Indeed, the kilonova that formed from the embers of GW170817 could have produced as much gold as the entire mass of the Earth – that is 1,000 trillion tonnes.</p>
<p><div data-react-class="Tweet" data-react-props="{"tweetId":"919936738893602816"}"></div></p>
<p>By observing a kilonova “up close and personal” for the very first time, and seeing how well it fits into the unfolding astronomical storyboard that began with the neutron star merger, astronomers have taken a huge leap forward in our understanding of these violent cosmic events. </p>
<p>The idea that we are all made of stardust is increasingly appreciated in popular culture – in everything from documentaries to song lyrics. But the mind-blowing concept that the gold in our wedding rings and Rolex watches is made of neutron stardust is about to catch on. Perhaps even more exciting, however, is the enormous potential now unlocked by this radical, new approach to studying the cosmos.</p>
<p>By working together collaboratively – using instruments that operate not just across the entire spectrum of light but are sensitive to gravitational waves and even neutrinos too – astronomers are poised to fully open a completely new “multi-messenger” window on the universe, with many further discoveries to be made and cosmic mysteries to be solved. For example, we have already used our observations to make the first ever joint measurement of the expansion rate of the universe, using both gravitational waves and light. Our paper will appear in Nature on October 16.</p>
<p>More results will also surely follow soon. The exciting new era of multi-messenger astronomy just started with a bang.</p><img src="https://counter.theconversation.com/content/85647/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Martin Hendry is a member of the LIGO scientific collaboration.</span></em></p>The discovery of tiny ripples in space from the violent collision of dense stars could help solve many mysteries – including where the gold in our jewellery comes from.Martin Hendry, Professor of Gravitational Astrophysics and Cosmology, University of GlasgowLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/810062017-07-14T09:14:31Z2017-07-14T09:14:31ZWe worked out what it would take to wipe out all life on a planet – and it’s good news for alien hunters<figure><img src="https://images.theconversation.com/files/178232/original/file-20170714-14287-1q8mu2h.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Shutterstock</span></span></figcaption></figure><p>The first exoplanet was spotted in 1988. Since then more than 3,000 planets <a href="https://theconversation.com/the-seven-most-extreme-planets-ever-discovered-78959">have been found</a> outside our solar system, and it’s thought that around 20% of Sun-like stars have an Earth-like planet in their habitable zones. We don’t yet know if any of these host life – and we don’t know how life begins. But even if life does begin, would it survive?</p>
<p>Earth has undergone at least five <a href="https://theconversation.com/five-mass-extinctions-and-what-we-can-learn-from-them-about-the-planet-today-79971">mass extinctions</a> in its history. It’s long been thought that an <a href="https://theconversation.com/how-does-an-invisible-underwater-crater-prove-an-asteroid-killed-the-dinosaurs-57711">asteroid impact ended the dinosaurs</a>. As a species, we are rightly concerned about events that could lead to our own elimination – climate change, nuclear war or disease could <a href="https://theconversation.com/the-five-biggest-threats-to-human-existence-27053">wipe us out</a>. So it’s natural to wonder what it would take to eliminate all life on a planet.</p>
<p>To establish a benchmark for this, we’ve been studying what is arguably the world’s hardiest species, the tardigrade, also known as the “water bear” for its appearance. <a href="https://www.nature.com/articles/s41598-017-05796-x">Our latest research </a> suggests these microscopic eight-legged creatures or their equivalents on other planets would be very hard to kill off on any planet that was like Earth. The only astrophysical catastrophes that could destroy them are so unlikely there’s an insignificant chance of them happening. This extreme survival ability adds weight to the idea that life is hardy enough to be found on other planets less hospitable than our own.</p>
<h2>Last survivors</h2>
<p>Tardigrades are known to survive incredible conditions. Drop the temperature briefly to <a href="https://link.springer.com/chapter/10.1007%2F978-94-007-1896-8_12">-272°C or raise it to 150°C and they go on.</a>
Increase atmospheric pressure to more than 1,000 times that at the Earth’s surface, or drop it to the <a href="http://www.cell.com/current-biology/fulltext/S0960-9822(08)00805-1?_returnURL=http%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0960982208008051%3Fshowall%3Dtrue">vacuum of space and they continue</a>. They can survive for up to <a href="http://www.sciencedirect.com/science/article/pii/S0011224015300134?via%3Dihub">30 years without food or water</a>. They can even <a href="https://www.newscientist.com/article/2106468-worlds-hardiest-animal-has-evolved-radiation-shield-for-its-dna/">withstand thousands of grays</a> (standard doses) of radiation. (Ten grays would be a lethal dose for most humans.)</p>
<p>They live all over the planet but can survive far below the ocean’s surface, around volcanic vents at the bottom of the <a href="https://theconversation.com/how-we-found-worlds-deepest-fish-in-the-mariana-trench-and-why-we-must-keep-exploring-35743">Mariana Trench</a> happily oblivious to the life and death of surface-dwelling mammals. Stripping the ozone layer or upper atmosphere would expose humans to lethal radiation but, at the bottom of the ocean, the water overhead would provide shielding.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/178230/original/file-20170714-3488-njpahi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/178230/original/file-20170714-3488-njpahi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=480&fit=crop&dpr=1 600w, https://images.theconversation.com/files/178230/original/file-20170714-3488-njpahi.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=480&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/178230/original/file-20170714-3488-njpahi.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=480&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/178230/original/file-20170714-3488-njpahi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=603&fit=crop&dpr=1 754w, https://images.theconversation.com/files/178230/original/file-20170714-3488-njpahi.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=603&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/178230/original/file-20170714-3488-njpahi.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=603&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Tardigrades: the last survivors?</span>
<span class="attribution"><span class="source">Shutterstock</span></span>
</figcaption>
</figure>
<p>We wanted to consider what cataclysmic events might be able to finally kill off the hardy tardigrade. What would need to happen to destroy every living thing on the planet? The simplest answer is that the planet’s entire oceans would have to boil. On Earth, this would require an incredible amount of energy –- 5.6 x 10<sup>26</sup> joules (around a million years’ of total human energy production at <a href="https://yearbook.enerdata.net/total-energy/world-energy-production.html">current rates</a>). We therefore have to consider the astrophysical events that could provide such an enormous amount of energy.</p>
<p>There are three primary candidates that could do this: asteroid impacts, supernovae and gamma-ray bursts. Of these, asteroids are the most familiar. We’ve been <a href="https://theconversation.com/ancient-asteroid-impacts-yield-evidence-for-the-nature-of-the-early-earth-59675">hit by several</a> over the course of Earth’s history. But, in our solar system there are just 17 candidate objects (including dwarf planets like Pluto and Eris) large enough to provide this energy – and none with orbits coinciding with that of Earth.</p>
<p>By looking at the rate of <a href="http://www.sciencedirect.com/science/article/pii/001910359290060K">asteroid impacts on Earth</a>, we can extrapolate the rate at which <a href="https://theconversation.com/could-asteroids-bombard-the-earth-to-cause-a-mass-extinction-in-ten-million-years-78937">doomsday events like this would likely occur</a>. This turns out to be approximately once every 10<sup>17</sup> years – far longer than the life of the universe. So it’s very, very unlikely to ever happen.</p>
<p>Supernovae (massive explosions of stars) <a href="http://iopscience.iop.org/article/10.1086/375682/meta">release huge amounts of energy</a> –- 10<sup>44</sup> joules, which is more than enough to <a href="https://www.nature.com/articles/s41598-017-05796-x">boil our oceans</a>. Fortunately, the energy delivered to a planet rapidly drops off the further away it is from a supernova. So for the Earth, sterilisation would require a supernova to occur within around <a href="https://www.nature.com/articles/s41598-017-05796-x">0.013 light-years</a>. The nearest star apart from the Sun, Proxima Centauri, is <a href="https://imagine.gsfc.nasa.gov/features/cosmic/nearest_star_info.html">4.25 light years away</a>(and is the wrong type to go supernova). </p>
<p>For Earth-like planets in our galaxy, the distance between stars depends on their distance from the galactic centre. The central bulge is more <a href="http://iopscience.iop.org/article/10.1086/321556/meta">densely populated than our neighbourhood.</a> But even closer in, given the rates at which supernovae occur, sterilisation is unlikely to happen more than once in 10<sup>15</sup> years, again far beyond the age of the universe.</p>
<p>Finally there are gamma-ray bursts, mysterious explosions producing enormous amounts of energy focused into jets of radiation as <a href="http://iopscience.iop.org/article/10.1086/519483">narrow as a couple of degrees</a>. Analysing these bursts as we did supernovae, we found that they could only kill off life on an Earth-like planet if their origin was within about 42 light-years and the planet lay within the beam. Again, the rate at which this would occur is sufficiently low that very few planets would ever be sterilised by a gamma-ray burst.</p>
<h2>Apocalypse never</h2>
<p>Given how tiny the chances are of any of these apocalyptic events actually happening, we’re left with the conclusion that tardigrades will survive until the Sun expands about 1 billion years from now. One final, incredibly unlikely possibility is that a passing star could kick a planet <a href="http://www.nature.com/news/2011/110518/full/news.2011.303.html">out of its orbit</a>. But, even then, volcanic vents that host some tardigrades could potentially provide heat for long enough for the planet to be captured by another star. </p>
<p>There are many events, both astrophysical and local, that could lead to the end of the human race. Life as a whole, however, is incredibly hardy. As we begin our search for life away from Earth, we should expect that if life had ever begun on a planet, some survivors might still be there.</p><img src="https://counter.theconversation.com/content/81006/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Rafael Alves Batista receives funding from the John Templeton Foundation. </span></em></p><p class="fine-print"><em><span>David Sloan receives funding from the John Templeton Foundation. </span></em></p>Hardy lifeforms such as tardigrades can survive almost anything.Rafael Alves Batista, Postdoctoral research associate, University of OxfordDavid Sloan, Postdoctoral research associate, University of OxfordLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/743722017-03-27T03:20:22Z2017-03-27T03:20:22ZSomething big exploded in a galaxy far, far away: what was it?<figure><img src="https://images.theconversation.com/files/162144/original/image-20170323-13491-1k8mr9u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">An artist's rendering of the Swift satellite catching a Gamma-ray Burst.</span> <span class="attribution"><a class="source" href="http://swift.sonoma.edu/resources/multimedia/images/">Nasa/Spectrum Astro </a></span></figcaption></figure><p>At 10:49pm Western Australian time on February 2 this year, cosmic gamma rays hit the NASA satellite, <a href="https://swift.gsfc.nasa.gov/">Swift</a>, orbiting the Earth. </p>
<p>Within seconds of the detection, an alert was automatically sent to the University of WA’s <a href="http://www.zt.science.uwa.edu.au/">Zadko Telescope</a>. It swung into robotic action, taking images of the sky location in the constellation Ophiuchus.</p>
<p>What emerged from the blackness, where nothing was seen before, was a rapidly brightening “optical transient”, which is something visible in the sky for a brief period of time.</p>
<p>The event, named GRB170202, was a very energetic gamma ray burst (<a href="https://theconversation.com/au/topics/gamma-ray-bursts-2295">GRB</a>). After less than a minute, the gamma rays switched off, and the GRB appeared as a brightening and then fading optical beacon. </p>
<p>The Zadko Telescope recorded the entire evolution of the optical outburst. During its biggest outburst, GRB170202 was equivalent in brightness to millions of stars shining together from the same location.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/161275/original/image-20170317-6100-1kn28x8.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/161275/original/image-20170317-6100-1kn28x8.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/161275/original/image-20170317-6100-1kn28x8.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/161275/original/image-20170317-6100-1kn28x8.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/161275/original/image-20170317-6100-1kn28x8.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/161275/original/image-20170317-6100-1kn28x8.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/161275/original/image-20170317-6100-1kn28x8.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/161275/original/image-20170317-6100-1kn28x8.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Zadko Telescope light curve of GRB170202, showing the evolving explosion and subsequent fading of the optical afterglow from seconds to hours after the gamma ray emission.</span>
<span class="attribution"><span class="source">Alain Klotz (Zadko collaboration)</span></span>
</figcaption>
</figure>
<p>About 9 hours 42 mins after the GRB, the <a href="http://www.eso.org/public/teles-instr/paranal-observatory/vlt/">Very Large Telescope</a> in Chile acquired the spectrum of the light from the optical afterglow. </p>
<p>This enabled a distance to the burst to be measured: about 12 billion light years. The Universe has expanded to four times the size it was then, 12 billion years ago, the time it took the light to reach Earth.</p>
<p>GRB170202 was so far away, even its host galaxy was not visible, just darkness. Because the GRB was a transient, never to be seen again, it is like turning on a light in a dark room (the host galaxy) and trying to record the detail in the room before the light goes out. </p>
<h2>Mystery of gamma ray burst</h2>
<p>The flash of gamma radiation and subsequent optical transient is the telltale signature of a black hole birth from the cataclysmic collapse of a star.</p>
<p>Such events are rare and require some special circumstances, including a very massive star up to tens of solar masses (the mass of our Sun) rotating rapidly with a strong magnetic field. </p>
<p>These ingredients are crucial to launch two jets that punch through the collapsing star to produce the gamma ray burst (<a href="https://www.nasa.gov/centers/goddard/mov/97789main_GRBstar2.mov">see animation</a>)</p>
<p>The closest analogue (and better understood transient) to a GRB is a <a href="https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-a-supernova.html">supernova</a> explosion from a collapsing star. In fact, some relatively nearby GRBs reveal evidence of an energetic supernova linked to the event.</p>
<p>Simulations show that most collapsing stars don’t have enough energy to produce a GRB jet, a so-called “failure to launch” scenario. Both observation and theory show that GRBs are extremely rare when compared to the occurrence of supernovae. </p>
<p>The stars that produce GRBs are born and die within some tens to hundreds of thousands of years, unlike our Sun which has been around for billions of years. </p>
<p>This is because very massive stars exhaust their fuel very quickly, and undergo violent gravitational collapse leading to a black hole, on the timescale of seconds.</p>
<h2>A plethora of rogue black holes</h2>
<p>The rates of black hole formation throughout the universe can be inferred from the GRB rate. Based on the observed GRB rate, there must be <a href="http://hubblesite.org/explore_astronomy/black_holes/encyc_mod3_q7.html">thousands of black hole births occurring each day</a> throughout the entire universe.</p>
<p>So what is the fate of these cosmic monsters? Most will be lurking in their host galaxies, occasionally devouring stars and planets. </p>
<p>Others will be in a gravitational death dance with other black holes until they merge into a single black hole with a burst of gravitational waves (GWs), such as the <a href="http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102">first discovery</a> of <a href="https://theconversation.com/gravitational-waves-discovered-the-universe-has-spoken-54237">such an event</a> by the Laser Interferometer Gravitational-Wave Observatory (LIGO).</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/161274/original/image-20170317-6109-1v2sdv3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/161274/original/image-20170317-6109-1v2sdv3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/161274/original/image-20170317-6109-1v2sdv3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/161274/original/image-20170317-6109-1v2sdv3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/161274/original/image-20170317-6109-1v2sdv3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/161274/original/image-20170317-6109-1v2sdv3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/161274/original/image-20170317-6109-1v2sdv3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/161274/original/image-20170317-6109-1v2sdv3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A binary black hole system seconds before merger.</span>
<span class="attribution"><span class="source">Frame from a visualisation of the binary black hole merger seen by LIGO [Visualisation by Simulating Extreme Spacetime (SXS) Collaboration]</span></span>
</figcaption>
</figure>
<h2>A new era</h2>
<p>At the frontier of understanding black hole formation is the search for a special kind of GRB that marks the merger (collision) of two neutron stars.</p>
<p>So called “<a href="https://science.nasa.gov/science-news/science-at-nasa/2008/20oct_briefmystery">short GRBs</a>” are flashes of gamma radiation that last less than a second and could be the “smoking gun” for neutron star mergers. </p>
<p>Importantly, merging neutron stars should be detected from their gravitational radiation by LIGO. Hence, a coincident detection in gamma rays, optical and gravitational waves is a real possibility. </p>
<p>This would be a monumental discovery allowing unprecedented insight into the physics of black hole formation. The revolution is like listening to the radio on a 1920s receiver and then watching a modern high definition surround sound movie.</p>
<h2>Future challenges</h2>
<p>Given the above rate of thousands of black holes created per day, it seems that coincident detection of GRBs and gravitational waves is a no brainer. </p>
<p>But in reality we must take into account the limited sensitivity of all the telescopes (and detectors). This reduces the potential observation rate to some tens per year. This is high enough to inspire a global scramble to search for the first coincident gravitational wave sources with electromagnetic counterparts. </p>
<p>The task is extremely difficult because the gravitational wave observatories cannot pinpoint the location of the source very well. To counter this, a strategy of searching for coincident gravitational wave and electromagnetic detections in time may be the best bet.</p>
<p>The newly funded ARC Centre of Excellence <a href="http://www.ozgrav.org/">OzGrav</a> mission is to understand the extreme physics of black holes. </p>
<p>One of the goals is to search for optical, radio and high energy counterparts coincident with gravitational waves from black hole creation. Australia is poised to play a significant role in this new era of “multi-messenger astronomy”.</p><img src="https://counter.theconversation.com/content/74372/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Coward receives partial funding support from the Australian Research Council Center of Excellence "OzGRav". The Zadko Telescope is supported by the University of Western Australia and OzGrav</span></em></p>A burst of gamma rays in a distant part of the universe reveals the birth of another black hole.David Coward, Associate professor, The University of Western AustraliaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/711782017-01-19T11:13:57Z2017-01-19T11:13:57ZSix cosmic catastrophes that could wipe out life on Earth<figure><img src="https://images.theconversation.com/files/153030/original/image-20170117-23036-q3go68.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A gamma ray burst close to Earth could be devastating.</span> <span class="attribution"><span class="source">ESO/A. Roquette</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>If you ask yourself what the biggest threat to human existence is you’d probably think of nuclear war, global warming or a large-scale pandemic disease. But assuming we can overcome such challenges, are we really safe?</p>
<p>Living on our blue little planet seems safe until you are aware of what lurks in space. The following cosmic disasters are just a few ways humanity could be severely endangered or even wiped out. Happy reading!</p>
<h2>1. High energy solar flare</h2>
<p>Our sun is not as peaceful a star as one might initially think. It creates strong magnetic fields that generate impressive sun spots, sometimes many times larger than Earth. It also ejects a stream of particles and radiation – the solar wind. If kept in check by Earth’s magnetic field, this wind can cause beautiful northern and southern lights. But when it becomes stronger, it can also influence radio communication or cause power outages.</p>
<p>The most powerful magnetic solar storm documented hit Earth in 1859. The incident, called the <a href="http://www.swsc-journal.org/articles/swsc/pdf/2013/01/swsc130015.pdf">Carrington Event</a>, caused huge interference with rather small scale electronic equipment. Such events must have happened several times in the past, too, with humans surviving. </p>
<p>But only in recent years have we become entirely dependent on electronic equipment. The truth is we would <a href="http://lasp.colorado.edu/home/wp-content/uploads/2011/07/lowres-Severe-Space-Weather-FINAL.pdf">suffer greatly</a> if we underestimate the dangers of a possible Carrington or even more powerful event. Even though this would not wipe out humanity instantly, it would represent an <a href="http://gizmodo.com/what-would-happen-if-a-massive-solar-storm-hit-the-eart-1724650105">immense challenge</a>. There would be no electricity, heating, air conditioning, GPS or internet – food and medicines would go bad.</p>
<h2>2. Asteroid impact</h2>
<p>We are now well <a href="https://asteroidday.org/">aware of the dangers asteroids could pose to humanity</a> – they are, after all, thought to have contributed to the extinction of the dinosaurs. Recent research has made us aware of the <a href="https://theconversation.com/now-thats-a-close-shave-secrets-of-the-halloween-asteroid-headed-for-earth-49820">large host of space rocks</a> in our solar system that could pose danger. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/153061/original/image-20170117-23050-10v9qpq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/153061/original/image-20170117-23050-10v9qpq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=454&fit=crop&dpr=1 600w, https://images.theconversation.com/files/153061/original/image-20170117-23050-10v9qpq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=454&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/153061/original/image-20170117-23050-10v9qpq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=454&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/153061/original/image-20170117-23050-10v9qpq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=571&fit=crop&dpr=1 754w, https://images.theconversation.com/files/153061/original/image-20170117-23050-10v9qpq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=571&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/153061/original/image-20170117-23050-10v9qpq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=571&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Dangerous impact.</span>
<span class="attribution"><span class="source">Don Davis/NASA</span></span>
</figcaption>
</figure>
<p>We are at the starting point of <a href="https://www.nasa.gov/planetarydefense/aida">envisaging and developing systems</a> for protecting us against some of the smaller asteroids that could strike us. But against the bigger and rarer ones we are quite helpless. While they would not always destroy Earth or even make it uninhabitable, they could wipe out humanity by causing enormous tsunamis, fires and other natural disasters.</p>
<h2>3. Expanding sun</h2>
<p>Where the previous cosmic dangers occur at the roll of a dice with a given probability, we know for certain that our sun will <a href="http://sro.sussex.ac.uk/1800/2/DoomsdayResub.pdf">end its life in 7.72 billion years</a>. At this point, it will throw off its outer atmosphere to form a planetary nebula, ending up as a stellar remnant know as a “white dwarf”. </p>
<p>But humanity will not experience these final stages. As the sun becomes older, it will become cooler and larger. By the time it becomes a <a href="https://theconversation.com/what-is-the-biggest-star-in-the-universe-52026">stellar giant</a> it will be big enough to engulf both Mercury and Venus. Earth might seem safe at this point, but the sun will also create an extremely strong solar wind that will slow down the Earth. As a result, in about 7.59 billion years, our planet will spiral into the outer layers of the hugely expanded dying star and <a href="http://sro.sussex.ac.uk/1800/2/DoomsdayResub.pdf">melt away forever</a>.</p>
<h2>4. Local gamma ray burst</h2>
<p>Extremely powerful outbursts of energy called <a href="http://www.bbc.co.uk/science/space/universe/sights/gamma_ray_bursts">gamma ray bursts</a> can be caused by binary star systems (two stars orbiting a common centre) and supernovas (exploding stars). These energy bursts are extremely powerful because they focus their energy into a narrow beam lasting no longer than seconds or minutes. The resulting radiation from one could damage and destroy our ozone layer, leaving life vulnerable to the sun’s harsh UV radiation. </p>
<p>Astronomers have discovered a star system – <a href="http://www.nature.com/articles/19033.epdf?referrer_access_token=tuD5AI9j4Eq3pLY2hFNl1tRgN0jAjWel9jnR3ZoTv0MRJSy94bsA9uUTZGJqAtFgRMWmmI-vrVpT7GkbP3NdIlLGAW8LnKKSbGobJd4nrg8I1yZlETA9q7-GM_JXc7c7KdYl64POgyOtdVTjJtjUQbmGY-3YQtAN9ZL-Nw9uKJu5wJ6o_1nNZRPh8g-Lrvs0osQ1A5NHgCYQDR8ZbiEkbk55V3SOahqSf_2ZDHqGWBc%3D&tracking_referrer=www.bbc.co.uk">WR 104</a> – that could host such an event. WR 104 is about 5,200-7,500 light years away, which is not far enough to be safe. And we can only guess when the burst will happen. Luckily, there is the possibility that the beam could miss us entirely when it does. </p>
<h2>5. Nearby supernovas</h2>
<p>Supernova explosions, which take place when a star has reached the end of its life, occur on average once or twice every 100 years in our Milky Way. They are more likely to occur closer to the dense centre of the Milky Way and we are about two-thirds of the way from the middle – not too bad.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/153065/original/image-20170117-23043-ty0qza.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/153065/original/image-20170117-23043-ty0qza.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/153065/original/image-20170117-23043-ty0qza.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/153065/original/image-20170117-23043-ty0qza.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/153065/original/image-20170117-23043-ty0qza.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/153065/original/image-20170117-23043-ty0qza.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/153065/original/image-20170117-23043-ty0qza.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">SN 1994D (bright spot on the lower left), a type Ia supernova in the NGC 4526 galaxy.</span>
<span class="attribution"><span class="source">NASA/ESA</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>So can we expect a nearby supernova anytime soon? The star Betelgeuse – a <a href="http://study.com/academy/lesson/red-supergiant-definition-facts-life-cycle.html">red super giant</a> <a href="http://www.space.com/22009-betelgeuse.html">nearing the end of its life</a> – in the constellation of Orion is just 460-650 light years away. It could become a supernova now or in the next million years. Luckily, astronomers have estimated that a supernova would need to be <a href="https://www.nasa.gov/topics/earth/features/2012-supernova.html">within at least 50 light years of us</a> for its radiation to damage our ozone layer. So it seems this particular star shouldn’t be too much of a concern.</p>
<h2>6. Moving stars</h2>
<p>Meanwhile, a <a href="http://iopscience.iop.org/article/10.1088/2041-8205/800/1/L17/meta">wandering star on its path through the Milky Way</a> might come so close to our sun that it would interact with the rocky “Oort cloud” at the edge of the solar system, which is the source of our comets. This might lead to an increased chance of a huge comet hurtling to Earth. Another roll of the dice.</p>
<p>The sun itself follows a <a href="https://www.newscientist.com/article/mg21228411-500-earths-wild-ride-our-voyage-through-the-milky-way/">path through the Milky Way</a> that takes us through more or less dense patches of interstellar gas. Currently we are within a <a href="https://arxiv.org/pdf/astro-ph/0401428v1.pdf">less dense bubble</a> created by a supernova. The sun’s wind and solar magnetic field help create a bubble-like region surrounding our solar system – <a href="https://en.wikipedia.org/wiki/Heliosphere">the heliosphere</a> – which shields us from interacting with the interstellar <a href="http://www.space.com/22729-voyager-1-spacecraft-interstellar-space.html">medium</a>. When we leave this region in 20,000 to 50,000 years (depending on current observations and models), our heliosphere could be less effective, exposing Earth. We would possibly <a href="https://www.newscientist.com/article/mg18524874-300-interstellar-gas-cloud-linked-to-snowball-earth/">encounter increased climate change</a> making life more challenging for humanity – if not impossible.</p>
<h2>And life goes on…</h2>
<p>The end of humanity on Earth is a given. But this is not something to make us crawl under a table. It is something that we cannot change, similar to our lives having a definite start and end. This is what defines us and makes us realise that the only thing we can do is make the most of our time on Earth. Especially when we know that Earth needs a careful balance to sustain humanity. </p>
<p>All the above scenarios harbour possible destruction, but in every instance they also offer beauty and wonder. In many cases, they produce what allowed us to be created. So rather than looking into the night sky and wondering what will kill us next, we should marvel at the depth of space, the wonders therein and the sublime nature of the universe. Be inspired by space. It offers future and meaning.</p><img src="https://counter.theconversation.com/content/71178/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Daniel Brown does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>If we survive for another 7.59 billion years, our planet will spiral into the outer layers of the dying sun and melt away forever.Daniel Brown, Lecturer in Astronomy, Nottingham Trent UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/348752014-12-16T05:17:15Z2014-12-16T05:17:15ZGamma ray bursts and biases: let’s stop fitting data to the model<figure><img src="https://images.theconversation.com/files/67310/original/image-20141216-24316-il18bz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">An artist’s conception of a of gamma ray burst.</span> <span class="attribution"><a class="source" href="http://www.eso.org/public/images/eso1418a/">ESO/NAOJ</a></span></figcaption></figure><p>Our understanding of gamma ray bursts (<a href="https://theconversation.com/flash-aah-aah-could-a-gamma-ray-burst-eradicate-all-life-on-earth-5291">GRBs</a>) – flashes of gamma rays from explosions in distant galaxies – since they were discovered more than 50 years ago may not be as solid as first thought. </p>
<p>Research by myself and colleagues, published in Monthly Notices of the Royal Society Letters and a <a href="http://arxiv.org/abs/1411.6711">preprint</a>, found that journals tended to publish papers which supported an already widely accepted model of how GRBs form, and reject those that don’t – a form of <a href="http://www.sciencedaily.com/articles/c/confirmation_bias.htm">confirmation bias</a>.</p>
<p>But our work shows that results that don’t conform to the <a href="http://ned.ipac.caltech.edu/level5/March04/Piran2/Piran9_3.html">standard model</a> still need to be considered. So can the standard model be salvaged in light of us uncovering the apparent confirmation bias?</p>
<p>NASA satellites detect an intense flash of gamma rays every few days, which are believed to originate from catastrophic explosions of massive stars (<a href="http://astronomy.swin.edu.au/cosmos/H/Hypernova">hypernovae</a>) in galaxies at the edge of the known universe. From Earth, telescopes triggered by NASA usually detect a rapidly fading afterglow following the prompt short flash.</p>
<p>The most popular model that describes the physics of GRB afterglow creation is the standard model (not to be confused with the <a href="https://theconversation.com/explainer-standard-model-of-particle-physics-2539">Standard Model</a> of Particle Physics). One core prediction of the GRB model is that the total energy of the high-energy flash determines the brightness of the fading afterglow. </p>
<p>The standard model has been successful in describing many individual GRBs and has remained firmly entrenched as the favoured and only viable model to explain observations. It predicts a significant correlation between the total GRB energy emitted and afterglow luminosity. </p>
<p>It is what has been previously observed and reported in numerous published studies, adding further support to the standard model.</p>
<p>But is it the only model? </p>
<h2>Other models</h2>
<p>An alternative – the <a href="http://arxiv.org/abs/0901.4260">Cannonball Model</a> – can explain several anomalous GRBs that don’t fit the standard model. One important concern that applies to both models is that fine-tuning has to be done to make either model work for individual GRBs.</p>
<p>Since 2007 I had my doubts about the standard model as it seemed to fall short when I considered GRB properties of the whole population of several hundred.</p>
<p>My main issue was that many peer-reviewed GRB studies used the growing number of GRB distance estimates to determine how they are distributed throughout the universe. This is important (remembering GRBs result from massive stars exploding) because it provides a way to indirectly see how massive stars are distributed across the universe.</p>
<p>There is one problem with this idea: the distances are obtained from a rapidly fading optical afterglow to obtain spectroscopic <a href="http://astronomy.swin.edu.au/cosmos/R/Redshift">redshift</a> so many distances cannot be measured because the afterglow is too dim.</p>
<p>This causes a bias (known as the <a href="http://en.wikipedia.org/wiki/Malmquist_bias">Malmquist Bias</a>) to measure only the brightest and nearest GRBs. This implies that one preferentially selects some data while inadvertently removing other data. </p>
<p>Importantly, the Malmquist Bias then produces a “false” correlation between intrinsic properties, such as total energy and luminosity. I soon realised that the intrinsic properties of GRBs that relied on distance measurements were biased, but now had to prove this.</p>
<h2>Rise to the challenge</h2>
<p>In 2013, armed with my growing scepticism and new insight, I was determined to test the reported GRB correlations.</p>
<p>I led a small team that began analysing the published studies that showed significant correlations of intrinsic properties of GRBs (using the standard model). Although some of these works did attempt to remove biases, I soon realised an obvious one remained. </p>
<p>The GRB afterglow luminosity data was distance dependent, introducing a Malmquist bias, which was not corrected for!</p>
<p>After re-analysing the data, but removing the afterglow Malmquist Bias, I found that previous reported correlations could not be reproduced and are in fact non-existent. Furthermore, I found that by including only the biased data, the correlation returned.</p>
<p>What this really means is that many scientists ignored or did not see fundamental biases in the data. Our work showed that previously reported correlations are an artifact of observation – nothing to do with the intrinsic properties of GRBs. </p>
<p>How could this happen in a rigorous peer reviewed culture, and why did I see this and not anyone else?</p>
<p>The answer is rooted in the psychology of scientists participating in a relatively volatile field (for astronomy anyway) dominated by several strong collaborative groups. The GRB standard model has become a powerful dogma that cannot easily be challenged, especially with no viable alternative. </p>
<p>So the previous studies naturally interpreted the observed GRB correlations as a result of the standard model. In general terms this behaviour is termed confirmation bias (also called confirmatory bias or myside bias). It is a tendency for people to favour information that confirms their preconceptions or hypotheses regardless of whether the information is true.</p>
<h2>Not perfect, but the best we have</h2>
<p>From my personal account, it is understandable that the public may be sceptical of the scientific process. But ultimately the work we did to uncover the truth was vindicated by a rigorous peer review (not without setbacks, as we had to find scientists not connected with the GRB standard model) and now <a href="http://arxiv.org/abs/1411.6711">published results</a>.</p>
<p>Despite the fact the scientific process is not perfect, it is the best system we have to converge to an informed understanding of the world around us. Sometimes this convergence is convoluted, as in my case, but ultimately science and society is a winner.</p>
<p>So what about the GRB standard model – is it in tatters? Actually, no. Its proponents are very good at fine-tuning the model to suit observations.</p>
<p>How much fine-tuning can you perform on a model before it should be abandoned? That is a question for another day.</p><img src="https://counter.theconversation.com/content/34875/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Coward receives funding from the Australian Research Council.</span></em></p>Our understanding of gamma ray bursts (GRBs) – flashes of gamma rays from explosions in distant galaxies – since they were discovered more than 50 years ago may not be as solid as first thought. Research…David Coward, ARC Future Fellow; Professor, The University of Western AustraliaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/272712014-05-28T06:26:05Z2014-05-28T06:26:05ZHeavens above! What made the cosmic flash that lit Earth today?<figure><img src="https://images.theconversation.com/files/49623/original/wmt6h7h5-1401256863.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The origin of today's burst of energy has astronomers puzzled.</span> <span class="attribution"><a class="source" href="http://www.flickr.com/photos/ap-photographie/8514447719">AP Photographie /Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span></figcaption></figure><p>A titanic eruption in our neighbouring galaxy, <a href="http://www.nasa.gov/mission_pages/chandra/multimedia/bonanza_image.html">Andromeda</a>, has sent shockwaves through the astronomical community here on Earth. </p>
<p>NASA’s <a href="http://swift.gsfc.nasa.gov/">Swift</a> satellite detected a flood of gamma rays at 21:15 UTC yesterday (7:15 AEST today), triggering telescopes across the globe within minutes to hunt for the afterglow of the explosion. Not long after, the news had raced around the world thanks to <a href="http://www.sciencealert.com.au/images/stories/PicMonkey_Collageastronomy.jpg.jpg">Twitter</a>. </p>
<p>While the cause of the explosion is a mystery at the moment, its implications could be huge.</p>
<p>There are many potential causes of such a flood of gamma rays, but the one that tops astronomers’ wish-list is a short-duration <a href="https://theconversation.com/death-of-a-star-how-radio-waves-can-capture-a-cosmic-obituary-822">gamma ray burst</a>. The cause is still unknown but current models suggest it’s an enormous collision (or inspiral) of two <a href="http://astronomy.swin.edu.au/cosmos/N/Neutron+Star">neutron stars</a>. These are the dead cores of massive stars with as much mass as the sun crushed into a region no larger than a city. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/49617/original/9q379c4j-1401254915.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/49617/original/9q379c4j-1401254915.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/49617/original/9q379c4j-1401254915.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/49617/original/9q379c4j-1401254915.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/49617/original/9q379c4j-1401254915.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/49617/original/9q379c4j-1401254915.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=425&fit=crop&dpr=1 754w, https://images.theconversation.com/files/49617/original/9q379c4j-1401254915.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=425&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/49617/original/9q379c4j-1401254915.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=425&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A neutron star compared to the island of Manhattan.</span>
<span class="attribution"><span class="source">NASA</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>During their inspiral and ultimate collision into a black hole, an enormous amount of energy is blasted out which we see as light of all wavelengths (from gamma rays, X-rays, visible/optical light and even into the radio).</p>
<p>The energies are so large that were such an explosion to occur in our galaxy it might lead to <a href="https://theconversation.com/flash-aah-aah-could-a-gamma-ray-burst-eradicate-all-life-on-earth-5291">mass extinctions on Earth</a>. Don’t worry though – the event in Andromeda is 2.5 million <a href="https://theconversation.com/explainer-light-years-and-units-for-the-stars-16995">light years</a> away making it harmless to us (and that this “breaking” story happened while humanity was barely in the Stone Age).</p>
<p>Such events will also set spacetime itself rippling, offering the hope of directly detecting Einstein’s final prediction of <a href="https://theconversation.com/topics/gravitational-waves">gravitational waves</a>.</p>
<figure>
<iframe src="https://player.vimeo.com/video/28966581" width="500" height="281" frameborder="0" webkitallowfullscreen="" mozallowfullscreen="" allowfullscreen=""></iframe>
</figure>
<p></p><p><a href="http://vimeo.com/28966581"></a><a href="</a"></a><a href="</a"></a></p><p></p><a href="</a">
<h2>The (un)usual suspects</h2>
<p>The most discussed cause for the explosion is a gamma ray burst, a collision between neutron stars. These are rare, with perhaps only one every million years expected in a galaxy such as ours or Andromeda.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/V2_kVIGdNRE?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The first 500 gamma ray bursts seen by NASA’s Swift satellite.</span></figcaption>
</figure>
</a><p><a href="</a">Other suggested causes could be a “belch” from a feeding black hole in what’s called a </a><a href="http://imagine.gsfc.nasa.gov/docs/science/know_l1/binary_stars.html">Low-Mass X-ray Binary</a>. This is unlikely as the energies are hundreds of time greater than what can normally be produced. </p>
<p>A similar idea, given a catch-all title of Ultraluminous X-ray (<a href="http://www.nasa.gov/mission_pages/swift/bursts/Ultraluminous-Xrays.html">ULX</a>) object, is that it’s a much larger black hole feeding messily, although likely we should have seen this before now.</p>
<p>Another possibility, and the one hardest to discriminate right now, is a flare from a <a href="https://theconversation.com/a-rare-magnetic-star-is-born-with-a-push-in-the-right-direction-26510">magnetar</a>, similar to the eruptions from our sun that cause the Northern and Southern Lights on Earth, but hugely more energetic. </p>
<p>These magnetars are super-magnetised neutron stars, with a magnetic field strong enough to wipe credit cards from half a million kilometres away. While plausible, the gamma ray signal comes from a clump of ancient stars, called a <a href="http://ned.ipac.caltech.edu/level5/ESSAYS/Cudworth/cudworth.html">Globular Cluster</a>, which would be unlikely to house a still powered-up magnetar.</p>
<h2>A new sense for humanity</h2>
<p>Everything we know about the world around us is via light or electromagnetic waves. Even when we touch objects the atoms never meet – instead we are stopped by electromagnetic fields between our hand and the object. </p>
<p>Yet if this explosion is a collision between two neutron stars then it would have opened up a fundamentally new sense for humanity, that of gravitational waves, entirely separate to electromagnetic waves. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/49619/original/qsj5dkqt-1401256050.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/49619/original/qsj5dkqt-1401256050.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/49619/original/qsj5dkqt-1401256050.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=422&fit=crop&dpr=1 600w, https://images.theconversation.com/files/49619/original/qsj5dkqt-1401256050.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=422&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/49619/original/qsj5dkqt-1401256050.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=422&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/49619/original/qsj5dkqt-1401256050.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=530&fit=crop&dpr=1 754w, https://images.theconversation.com/files/49619/original/qsj5dkqt-1401256050.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=530&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/49619/original/qsj5dkqt-1401256050.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=530&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The LIGO facility in California.</span>
<span class="attribution"><span class="source">NASA</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Although indirect detections have been <a href="https://theconversation.com/first-hints-of-gravitational-waves-in-the-big-bangs-afterglow-24475">claimed</a>, today could have been the first direct confirmation in the lab. </p>
<p>Such a detection would be a potential Nobel Prize winning discovery, which is why it’s particularly unlucky that the Laser Interferometer Gravitational Wave Observatory (<a href="http://www.ligo.org/science/GW-Inspiral.php">LIGO</a>) facility built to detect these gamma ray bursts is currently shutdown for an upgrade.</p>
<h2>After the excitement</h2>
<p>Over the next few days the manner in which the afterglow fades will tell us if it’s the hoped-for gamma ray burst or less extreme but no less exotic magnetar flares or feeding black holes. </p>
<p>In addition to telescopes, particle detectors such as the <a href="http://icecube.wisc.edu/">IceCube</a> facility in Antarctica will be searching for any hints of ghost-like neutrino particles that are often associated with high energy events in the sky, although recent efforts show gamma ray bursts unusually <a href="https://theconversation.com/an-extragalactic-mystery-where-do-high-energy-cosmic-rays-come-from-6623">might be an exception</a>. </p>
<p>Finally, operating gravitational wave detectors, such as <a href="http://www.geo600.org/">GEO600</a>, will analyse their data for a tell-tale high frequency “chirp” as the two neutron stars orbited each other ever faster until finally colliding.</p>
<p>Whatever the cause of the explosion ultimately is, the events of today have shown that even in a science that measures time in billions of years, things can still move fast in astronomy.</p>
<hr>
<p><em>Update: an announcement from the NASA Swift team was that this completely unexpected signal from Andromeda was a <a href="http://gcn.gsfc.nasa.gov/gcn3/16336.gcn3">false alert</a> after all.</em></p>
<p><em>The signal from an existing X-ray source, added to the normal random extra X-rays from distant objects, pushed the NASA satellite over its threshold and triggered an automatic email to astronomers worldwide. Some excellent reanalysis by the team discovered the mistake within a day.</em></p>
<p><em>In science, just because you want a result doesn’t mean nature will oblige, but one thing is for sure – with a cosmic explosion once a day it’s only a matter of time before we get to see the most violent event in the universe close up.</em></p><img src="https://counter.theconversation.com/content/27271/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Alan Duffy does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>A titanic eruption in our neighbouring galaxy, Andromeda, has sent shockwaves through the astronomical community here on Earth. NASA’s Swift satellite detected a flood of gamma rays at 21:15 UTC yesterday…Alan Duffy, Research Fellow, Swinburne University of TechnologyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/157002013-07-04T06:14:01Z2013-07-04T06:14:01ZFast Radio Bursts: new intergalactic messengers<figure><img src="https://images.theconversation.com/files/26737/original/927dhq8n-1372805756.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C2048%2C1152&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist's composite of the CSIRO's 64m Parkes Radio Telescope showing an extragalactic radio burst appearing briefly, far from the Milky Way's disk.</span> <span class="attribution"><span class="source">CSIRO/Harvard/Swinburne Astronomy Productions</span></span></figcaption></figure><p>How many electrons are there in the universe? That may seem nigh on impossible to calculate – let alone comprehend – but the discovery of a new population of astrophysical events called Fast Radio Bursts (FRBs), <a href="http://www.sciencemag.org/content/341/6141/53">published in Science today</a> by my colleagues and I, could help provide a solution to this fundamental cosmological question.</p>
<p>The FRBs, discovered with the CSIRO’s Parkes radio telescope in New South Wales, confirm the validity of the amazing “<a href="http://astronomy.swin.edu.au/cosmos/E/Extragalactic+Radio+Bursts">Lorimer burst</a>”, a mysterious event that occurred in 2001 and has until now been quite controversial.</p>
<p>The techniques for finding FRBs closely mirror those used to discover <a href="http://astronomy.swin.edu.au/cosmos/p/pulsar">pulsars</a>, sources of radio emission emitted in lighthouse-like beams first discovered in 1967 by the Northern-Irish astrophysicist <a href="http://en.wikipedia.org/wiki/Jocelyn_Bell_Burnell">Jocelyn Bell</a>.</p>
<p>Though we don’t yet know for sure, it’s worth pointing out that FRBs are probably caused by some catastrophic event in the distant universe. They almost certainly occur in galaxies and probably involve relativistic objects such as <a href="https://theconversation.com/explainer-black-holes-7431">black holes</a> or <a href="http://astronomy.swin.edu.au/cosmos/N/Neutron+Star">neutron stars</a>. </p>
<p>Some candidates are millisecond-duration explosions on the surface of ultra-magnetic neutron stars known as <a href="http://astronomy.swin.edu.au/cosmos/M/Magnetar">magnetars</a>, coalescing neutron stars or <a href="http://astronomy.swin.edu.au/cosmos/S/Supernova">supernova explosions</a>.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/V5p6K6QTJsc?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Finding – and measuring – Fast Radio Bursts.</span></figcaption>
</figure>
<h2>FRB cousins: the gamma-ray bursts</h2>
<p>One of the greatest discoveries of the modern astrophysical era were the <a href="https://theconversation.com/flash-aah-aah-could-a-gamma-ray-burst-eradicate-all-life-on-earth-5291">gamma ray bursts</a>, or GRBs. GRBs are associated with exploding stars and have allowed astronomers to see back until the universe was only a small fraction of its current age. They are short (few second) bursts of gamma-rays that occur infrequently and appear to be randomly distributed on the sky.</p>
<p>The <a href="http://astronomy.swin.edu.au/cosmos/G/Gamma+Ray+Burst+History">first GRBs</a> were found by satellites designed to detect nuclear tests during the Cold War, and to the shock of the engineers involved, seemed to indicate that nuclear tests were being conducted in space!</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/26837/original/984jdy4c-1372894613.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/26837/original/984jdy4c-1372894613.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/26837/original/984jdy4c-1372894613.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=447&fit=crop&dpr=1 600w, https://images.theconversation.com/files/26837/original/984jdy4c-1372894613.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=447&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/26837/original/984jdy4c-1372894613.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=447&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/26837/original/984jdy4c-1372894613.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=562&fit=crop&dpr=1 754w, https://images.theconversation.com/files/26837/original/984jdy4c-1372894613.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=562&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/26837/original/984jdy4c-1372894613.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=562&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">An artist’s impression of a supernova explosion.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>It took more than 20 years for astronomers to ascertain what the GRBs were, but the mystery was eventually (at least partially) solved when a nearby GRB was associated with a supernova explosion. This led to their correct interpretation and their extreme luminosity has enabled astronomers to learn not only about what the GRBs are, but also about the evolution of the universe.</p>
<h2>Getting to know you</h2>
<p>The FRBs are not (yet) associated with GRBs, but have some similarities. </p>
<p>They are short duration (a few milliseconds) bursts of radio emission that arrive first at high frequencies and then progressively sweep to lower frequencies before disappearing completely and not recurring. The sweep from high radio frequencies to low is called “<a href="http://astronomy.swin.edu.au/cosmos/P/Pulsar+Dispersion+Measure">pulse dispersion</a>”, and has a characteristic form.</p>
<p>On their approximately 10-billion-year intergalactic voyage, every time the radio waves pass an electron they are delayed in a frequency-dependent manner that ultimately amounts to about a one second difference across our observing frequency range. This delay, shown in the figures below, allows us to “count” the number of electrons between us and the burst. </p>
<p>The beautiful sweep and pulse shape of the radiation obeys the theoretically-predicted relation for a burst of radio waves that has propagated across the universe.</p>
<p>Since we think most of the atoms in the universe have lost their electrons, counting them gives us a fundamental insight into the amount of normal or “<a href="http://en.wikipedia.org/wiki/Baryon#Baryonic_matter">baryonic</a>” matter in it. Like the GRBs before them, at present we have no solitary idea of what the FRBs are, and like the GRBs they appear to come from great distances, such as halfway across the observable universe.</p>
<p>A population of FRBs at different stages of the universe’s age would be an invaluable resource, allowing us to map the evolution of the mass distribution. They’re potentially a cosmological gold mine!</p>
<p>But the history of the FRBs has not been without controversy. The first example of an FRB was the famous “Lorimer burst”, mentioned at the outset of this article and <a href="http://www.sciencemag.org/content/318/5851/777.abstract">reported in Science in 2007</a> by astrophysicist Dunc Lorimer and colleagues.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/26596/original/g4pn6t6c-1372655545.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/26596/original/g4pn6t6c-1372655545.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/26596/original/g4pn6t6c-1372655545.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=462&fit=crop&dpr=1 600w, https://images.theconversation.com/files/26596/original/g4pn6t6c-1372655545.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=462&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/26596/original/g4pn6t6c-1372655545.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=462&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/26596/original/g4pn6t6c-1372655545.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=581&fit=crop&dpr=1 754w, https://images.theconversation.com/files/26596/original/g4pn6t6c-1372655545.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=581&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/26596/original/g4pn6t6c-1372655545.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=581&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The ‘waterfall plot’ of the famous ‘Lorimer burst’ showing the characteristic dispersion sweep expected from extragalactic pulses of radio radiation that have travelled cosmological distances. The inset shows the pulse after adding all of the frequency channels and correcting for the sweep.</span>
</figcaption>
</figure>
<p>Unlike most controversial results, the Lorimer burst’s problem was not that it was too weak, but rather that it was too bright! Fainter, more distant analogues should be seen, and none were, despite large international efforts.</p>
<p>Interest in the subject waned - until last year.</p>
<h2>The High Time Resolution Universe Surveys</h2>
<p>Since most pulsars live in our galaxy, pulsar astronomers invariably hunt for them in the plane of the Milky Way; once this area is exhausted they reluctantly move to high galactic latitudes.</p>
<p>The surveys our international team have been conducting at the Parkes 64-metre telescope were no exception.</p>
<p>In mid 2012, University of Manchester PhD student Dan Thornton started looking intensively through the results of the relatively boring “off plane” regions for pulsars, rotating radio transients (short radio pulses), and just maybe, Lorimer bursts.</p>
<p>Dan was somewhat daunted by the presence of radio interference, and his supervisor Ben Stappers suggested he just “set a high threshold” and see if there was anything interesting in his data.</p>
<p>Shortly afterwards he discovered the first burst - FRB 110220 shown below - an amazingly bright pulse and with a dispersion delay some three times that of the Lorimer burst!</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/26601/original/dzj9hg74-1372657942.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/26601/original/dzj9hg74-1372657942.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/26601/original/dzj9hg74-1372657942.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=466&fit=crop&dpr=1 600w, https://images.theconversation.com/files/26601/original/dzj9hg74-1372657942.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=466&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/26601/original/dzj9hg74-1372657942.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=466&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/26601/original/dzj9hg74-1372657942.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=585&fit=crop&dpr=1 754w, https://images.theconversation.com/files/26601/original/dzj9hg74-1372657942.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=585&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/26601/original/dzj9hg74-1372657942.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=585&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Waterfall plot of the fast radio burst FRB 110220 discovered by Dan Thornton (University of Manchester). The image shows the power as a function of time (x axis) for more than 800 radio frequency channels (y axis) and shows the characteristic sweep one expects for sources of galactic and extragalactic origin. In the inset the pulse is shown after summing all the channels and correcting for the dispersion ‘sweep’. The fast rise and exponential decay is characteristic of a pulse scattered by free electrons en route to Earth and both the sweep and scatter tail are compelling evidence of the pulse’s extragalactic origins.</span>
<span class="attribution"><span class="source">Matthew Bailes</span></span>
</figcaption>
</figure>
<p>Director of Germany’s <a href="http://www.mpifr-bonn.mpg.de/2169/en">Max Planck Institute for Radio Astronomy</a> <a href="http://www3.mpifr-bonn.mpg.de/staff/mkramer/About_Me.html">Michael Kramer</a> demonstrated with our new digital equipment the pulse’s dispersion (time delay) sweep could be shown to be perfectly consistent with the theoretically-expected relation, but we still had the problem that we should be seeing other, still weaker, bursts. </p>
<p>Dan soon uncovered two fainter bursts, and then a third, and our confidence in them grew.</p>
<p>The four bursts published in today’s Science paper are certainly consistent with them being part of the same population as the Lorimer burst. The telescope’s observed field of view and event rate suggest that every day, a few thousand similar radio bursts strike Earth; however, we have the instrumentation to detect only one every week or two of observing time, and only when far from the galactic plane.</p>
<p>If we can get more accurate positions, the FRBs will be arguably one of the most important cosmological probes we possess. The combination of a host galaxy <a href="https://theconversation.com/explainer-the-doppler-effect-7475">redshift</a> and the dispersion time provides the mass of normal matter in the universe in a completely new way. </p>
<h2>Origins</h2>
<p>From here, astronomers will pursue the joint aims of determining the origin of the bursts and using them to gain new insights into the universe.</p>
<p>Our team now have a real-time burst detector operating at the <a href="http://astronomy.swin.edu.au/cosmos/P/Parkes+64+metre+radio+telescope">Parkes telescope</a> and are hoping to refurbish the University of Sydney’s old Molonglo radio telescope near Canberra to be a dedicated FRB finder. Looking further afield, the <a href="http://www.skatelescope.org/">Square Kilometre Array</a> and its pathfinders should detect these bursts at rapid rates and allow us to determine their host galaxies.</p>
<p>Last night we found a new event, and the hunt for the origin and physical understanding of these new intergalactic messengers is sure to continue at a furious pace.</p>
<p><br>
<strong>Acknowledgements:</strong></p>
<p><em>I am indebted to the entire HITRUN team [D. Thornton, B.Stappers, B. Barsdell, S. Bates, N. D. R. Bhat, M. Burgay, S. Burke-Spolaor, D. Champion, P. Coster, N. D'Amico, A. Jameson, S. Johnston, M. Keith, M. Kramer, L. Levin, S. Milia, C. Ng, A. Possenti, W. van Straten] and their host institutions, the Universities of Manchester, Swinburne, Curtin, and WVU, INAF, MPI for Radio Astronomy, JPL/Caltech and CSIRO’s CASS division. Special thanks to Dan Thornton for his Jocelyn Bell-like diligence and patience and also to Dunc Lorimer for involving me in the original Lorimer burst discovery and follow-ups. This research is supported in Australia by the ARC and CSIRO’s Astronomy and Space Science division. Matthew Bailes is the Dynamic Universe theme leader of the ARC Centre of Excellence for All-sky Astrophysics “CAASTRO”.</em></p><img src="https://counter.theconversation.com/content/15700/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Matthew Bailes receives funding from the Australian Research Council and Swinburne University of Technology to conduct the research in this piece. He works for Swinburne University of Technology and is the leader of the Dynamic Universe theme of the ARC Centre of Excellence for all-sky astrophysics "CAASTRO".
</span></em></p>How many electrons are there in the universe? That may seem nigh on impossible to calculate – let alone comprehend – but the discovery of a new population of astrophysical events called Fast Radio Bursts…Matthew Bailes, Pro-Vice Chancellor (Research) , Swinburne University of TechnologyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/66232012-04-23T20:29:54Z2012-04-23T20:29:54ZAn extragalactic mystery: where do high-energy cosmic rays come from?<figure><img src="https://images.theconversation.com/files/9853/original/9vpmjr5j-1335158346.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Knowing where cosmic rays don't come from brings scientists another step closer to determining their origin.</span> <span class="attribution"><span class="source">NSF/J. Yang</span></span></figcaption></figure><p>It’s been the defining question of high-energy astrophysics for the past century: where do <a href="http://en.wikipedia.org/wiki/Cosmic_ray">cosmic rays</a> come from?</p>
<p>New findings from the <a href="http://icecube.wisc.edu/">IceCube Neutrino Observatory</a> at the South Pole have brought us closer to understanding the origin of these strange “rays” – charged particles that originate somewhere beyond our planet and that reach Earth with varying energy levels and in different forms.</p>
<p>A <a href="http://icecube.wisc.edu/news/view/54">recent announcement</a> by IceCube scientists suggests the newly constructed observatory has found evidence allowing them to <a href="http://www.abc.net.au/science/articles/2012/04/19/3480435.htm">rule out gamma-ray bursts</a> – the most energetic explosions in the known universe – as the most likely source of the highest-energy cosmic rays.</p>
<p>This is big news, make no mistake. But to understand the significance of this finding, we first need to take a look at the history of cosmic-ray research.</p>
<h2>Cosmic(-ray) history</h2>
<p>Some 100 years ago, the pioneering Austrian physicist <a href="http://www.mpi-hd.mpg.de/hfm/HESS/public/hessbio.html">Victor Hess</a> jumped in a hot-air balloon and climbed to the dizzying height of 5.3km above the earth’s surface.</p>
<p>Why? Because he, as with many scientists of his day, was wondering where exactly <a href="http://en.wikipedia.org/wiki/Ionizing_radiation">ionising radiation</a> was coming from.</p>
<p>Ionising radiation is comprised of particles that can react with matter; radiation such as that seen in the fallout from the <a href="https://theconversation.com/topics/fukushima">Fukushima nuclear disaster</a>.</p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/9865/original/wh9ygs97-1335176090.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/9865/original/wh9ygs97-1335176090.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=1040&fit=crop&dpr=1 600w, https://images.theconversation.com/files/9865/original/wh9ygs97-1335176090.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=1040&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/9865/original/wh9ygs97-1335176090.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=1040&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/9865/original/wh9ygs97-1335176090.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1307&fit=crop&dpr=1 754w, https://images.theconversation.com/files/9865/original/wh9ygs97-1335176090.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1307&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/9865/original/wh9ygs97-1335176090.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1307&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Victor Hess’s balloon trip was a foundation moment in the study of cosmic rays.</span>
</figcaption>
</figure>
<p>(It is important to understand, however, that the ionising radiation that was a mystery in the early 20th century is stopped by the Earth’s atmosphere. It was the by-products of the radiation’s interaction with the atmosphere that was being observed.)</p>
<p>Hess’s major achievement was to show that as altitude increases, so do the levels of ionising radiation – and so the radiation must have been coming from space. This radiation was thus given the title “cosmic rays” and Hess was awarded the 1935 Nobel Prize in Physics for his troubles.</p>
<p>Ever since, scientists such as myself have been wondering where these cosmic rays actually came from.</p>
<h2>What we know</h2>
<p>While we are still in the dark about some major aspects of the origins of cosmic rays, we’ve made considerable progress.</p>
<p>We know cosmic rays are made up of charged particles: primarily <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/particles/proton.html">protons</a> (roughly 89%), helium (roughly 9%) and <a href="http://www.chem4kids.com/files/atom_electron.html">electrons</a> (roughly 1%) with heavier elements (up to iron) making up the other 1%.</p>
<p>We also know cosmic rays arrive at Earth with an impressive energy range: from millions of electron volts – an <a href="http://en.wikipedia.org/wiki/Electron_volt">electron volt (eV)</a> is the amount of energy gained by the charge of a single electron when it is moved across an <a href="http://en.wikipedia.org/wiki/Electric_potential">electric potential difference</a> of one volt – up to more than 10<sup>21</sup> eV (the number 1 with 21 zeroes after it!).</p>
<p>These 10<sup>21</sup> eV particles are the most energetic particles ever observed – subatomic particles with the energy of a vigorous tennis serve!</p>
<p>Given the huge energy range with which cosmic rays arrive at Earth, it seems clear we need several explanations for their origin.</p>
<h2>Some from the sun</h2>
<p>The sun supplies many of the low-energy cosmic rays seen on Earth, while sources within our galaxy probably make up most of the rest. The cosmic rays at intermediate energies are probably created through the explosion of dead stars (known as <a href="http://www.mso.anu.edu.au/%7Ebrian/PUBLIC/supernovae.html">supernovae</a>), their remnants and other detritus from their lives.</p>
<p>But there are still large gaps in our knowledge. There is precious little evidence about the exact site(s) of cosmic-ray acceleration – cosmic rays being sped up to extremely high energies – above what the sun or the Milky Way can produce.</p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/9867/original/8ngcz4mv-1335176526.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/9867/original/8ngcz4mv-1335176526.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/9867/original/8ngcz4mv-1335176526.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=720&fit=crop&dpr=1 600w, https://images.theconversation.com/files/9867/original/8ngcz4mv-1335176526.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=720&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/9867/original/8ngcz4mv-1335176526.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=720&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/9867/original/8ngcz4mv-1335176526.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=905&fit=crop&dpr=1 754w, https://images.theconversation.com/files/9867/original/8ngcz4mv-1335176526.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=905&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/9867/original/8ngcz4mv-1335176526.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=905&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The lowest-energy cosmic rays (yellow band) come from the sun, intermediate-energy cosmic rays (blue band) originate in our galaxy while the highest-energy cosmic rays (purple band) are extragalactic in origin.</span>
<span class="attribution"><span class="source">Wikimedia Commons</span></span>
</figcaption>
</figure>
<p>At modest energies, there is <a href="http://www.nature.com/nature/journal/v378/n6554/abs/378255a0.html">very good evidence</a> – from X-ray and radio telescopes – that the remnants of supernovae can accelerate cosmic-ray electrons. But we still have no strong evidence where cosmic-ray protons, which, you’ll recall, make up roughly 90% of cosmic rays, are accelerated, even in our own cosmic backyard.</p>
<p>And even after a century of intense study, we still don’t know the exact source of the highest-energy cosmic rays; subatomic particles with energies above about 10<sup>15</sup> eV. These particles are found above what is known as the “knee” of the cosmic ray spectrum (see image above) and are thought to come from extragalactic sources.</p>
<p>So, how do we go about determining the source of these high-energy cosmic rays, observationally? There are two methods: direct and indirect detection.</p>
<h2>Direct detection of cosmic rays</h2>
<p>Due to the fact cosmic rays are made up of charged particles, they are – unlike <a href="http://en.wikipedia.org/wiki/Photon">photons</a> (light particles) – deflected by ambient magnetic fields. As a result, the exact location of their acceleration is lost.</p>
<p>But the relative weakness of galactic and intergalactic magnetic fields implies this does not occur for sufficiently energetic cosmic rays – rays with energy levels above 10<sup>19</sup> eV.</p>
<p>Thus it was a great breakthrough in 2007 when the <a href="http://www.auger.org/">Pierre Auger Observatory</a> – a vast array of tanks and fluorescence (light) detectors dedicated to directly detecting cosmic rays at the highest energies – reported there was a <a href="http://arxiv.org/abs/0712.2843">statistically significant correlation</a> between cosmic rays and <a href="http://en.wikipedia.org/wiki/Active_galactic_nucleus">active galactic nuclei</a> – the centres of galaxies which contain supermassive black holes.</p>
<p>In other words, it appeared as if high-energy cosmic rays were coming from active galactic nuclei.</p>
<p>Unfortunately, this evidence is getting worse. While scientists at the Pierre Auger Observatory were sure their data were correct and their result was <a href="http://www.abc.net.au/science/articles/2012/04/11/3474740.htm">statistically significant</a>, in statistics, nothing is certain.</p>
<p>A small number of results will actually be a meaningless blip and, unfortunately for the researchers at the Pierre Auger Observatory, this result was one of them. Their signal is, sadly, disappearing into the statistical muck of random noise.</p>
<p>Therefore, for now, we need a non-interacting proxy for detecting cosmic rays: the <a href="https://theconversation.com/explainer-the-elusive-neutrino-431">neutrino</a>.</p>
<h2>Indirect detection of cosmic rays</h2>
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<figcaption>
<span class="caption">One of two large columns containing cables for the IceCube experiment.</span>
<span class="attribution"><span class="source">Michael Ashley</span></span>
</figcaption>
</figure>
<p>Neutrinos are almost-massless, charge-less particles (their name means “little neutral one”) which are created in many particle interactions. You might remember that neutrinos were at the centre of the <a href="https://theconversation.com/neutrinos-and-the-speed-of-light-not-so-fast-3513">faster-than-light kerfuffle</a> earlier this year.</p>
<p>The neutrino’s lack of charge and mass, however, makes them incredibly difficult to detect and as a result, neutrino detectors have to be enormous.</p>
<p>As it turns out, one of the best detection mediums is the ice of Antarctica and so, to detect neutrinos from outer space, the one-cubic-kilometre IceCube observatory was built at the South Pole.</p>
<h2>The gamma-ray burst link</h2>
<p>For the vast majority of the past century, without enormous detectors such as IceCube, the only source of information about the acceleration of the highest-energy cosmic rays was purely theoretical.</p>
<p>Many of these theories have posited that mysterious bursts of gamma rays (photons with very high energies), called <a href="https://theconversation.com/flash-aah-aah-could-a-gamma-ray-burst-eradicate-all-life-on-earth-5291">gamma-ray bursts</a>, could provide a meaningful <a href="http://en.wikipedia.org/wiki/Flux">flux</a> of particles at the highest energies.</p>
<p>Gamma-ray bursts, the most energetic events in the known universe, are observed at a rate of a few per galaxy per century. They last from milliseconds to minutes and are followed by afterglows emitted at <a href="http://en.wikipedia.org/wiki/Electromagnetic_spectrum">X-ray to radio wavelengths</a>.</p>
<p>Despite their rarity, the vast expanse of our universe makes (or according to IceCube’s current research, made) these events a great potential source for the highest-energy cosmic rays.</p>
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<span class="caption">Recent results suggest gamma-ray bursts aren’t responsible for creating high-energy cosmic rays.</span>
<span class="attribution"><span class="source">NASA (artist's conception)</span></span>
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</figure>
<p>We know that neutrinos are produced in the same interactions that should produce the gamma rays that give gamma-ray bursts their name. Hence, the theory goes, neutrinos should be observed from gamma-ray bursts.</p>
<p>Given this, it leaves particle physics in a precarious position, given that neutrinos from these gamma-ray bursts are not seen by IceCube.</p>
<h2>The importance of no neutrinos</h2>
<p>There are two schools of thought about the sources of cosmic rays at the highest energies. Either they’re created bottom-up – accelerated from lower energies, such as in gamma-ray bursts – or top-down – where more-energetic particles interact in some way and lose energy, creating cosmic rays.</p>
<p>Top-down models usually posit the existence of exotic <a href="https://theconversation.com/topics/dark-matter">dark matter</a> particles (although <a href="http://www.eso.org/public/news/eso1217/">recent results</a> suggest that even dark matter is not on as certain scientific footing as it once was).</p>
<p>The crux of the problem, really, is that other than active galactic nuclei and gamma-ray bursts, there are no known, credible sources that are abundant enough to create these high-energy cosmic rays.</p>
<p>This fact leads us to a startling conclusion: either the IceCube data is wrong (as with the Pierre Auger Observatory results, we might have seen a random fluctuation of the data masquerading as a statistically significant result), our understanding of gamma-ray bursts is wrong, or we have to appeal to a completely new paradigm of physics.</p>
<p>In this new paradigm, cosmic rays at the highest energies might be created by unknown particles, (possibly) under the influence of novel particle physics.</p>
<p>This could lead to this century’s Albert Einstein or Victor Hess showing the rest of us what we’ve been missing this whole time.</p>
<p>One thing’s for sure: this is a very exciting time to be a particle physicist. </p><img src="https://counter.theconversation.com/content/6623/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Jones does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>It’s been the defining question of high-energy astrophysics for the past century: where do cosmic rays come from? New findings from the IceCube Neutrino Observatory at the South Pole have brought us closer…David Jones, Postdoctoral Fellow in High Energy Astro-particle physicsLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/52912012-02-20T13:28:44Z2012-02-20T13:28:44ZFlash, aah-aah! Could a gamma ray burst eradicate all life on Earth?<figure><img src="https://images.theconversation.com/files/7578/original/cwxzt5pz-1329087431.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">GRBs have puzzled astronomers for decades, and there is still plenty to learn.</span> <span class="attribution"><span class="source">EOS/A Roquette</span></span></figcaption></figure><p>Ever since they were discovered accidentally in the 1960s, <a href="https://theconversation.com/death-of-a-star-how-radio-waves-can-capture-a-cosmic-obituary-822">gamma ray bursts (GRBs)</a> have continued to amaze and puzzle astronomers worldwide. In nearly 50 years of research there seem to have been more theories trying to explain GRBs then there have been actual GRBs.</p>
<p>So just what are GRBs? Where do they come from? And what would happen if one occurred in our cosmic backyard?</p>
<p>GRBs were first detected by US military satellites in the 1960s and were initially thought to be violations of a treaty that was created to prevent the testing of nuclear devices in space – the <a href="http://en.wikipedia.org/wiki/Partial_Nuclear_Test_Ban_Treaty">Partial Nuclear Test Ban Treaty</a>.</p>
<p>The <a href="http://heasarc.nasa.gov/docs/heasarc/missions/vela5a.html">US Vela satellites</a> were launched in 1962 to enforce this treaty, by monitoring space for the flashes of gamma rays emitted from nuclear explosions. </p>
<p>Gamma ray flashes were detected regularly by the Vela satellites, and subsequently by USSR satellites. The question everyone wanted answered was: who was detonating nuclear explosions in space close to Earth?</p>
<p>The answer, of course, was “no-one” and, as the Vela satellites observed ever-more GRBs, it became clear the flashes weren’t coming from Earth or the sun either.</p>
<p>In the 1970s the mysterious flashes were “declassified” by the US government for the first time, and the data was unleashed on the astronomical research community in the hope of finding an answer. It wasn’t long until all kinds of theories started to emerge.</p>
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</figure>
<p>Mal Ruderman, Columbia University physicist, said at a conference in 1974 that: “there are more theories than bursts”. Some of the exotic theories included collisions between comets and <a href="http://en.wikipedia.org/wiki/Neutron_star">neutron stars</a> in our galaxy, and even communications from aliens. </p>
<p>The disputes and arguments continued throughout the 1980s, but the general consensus was that GRBs originate from somewhere within our own galaxy, the Milky Way. </p>
<p>The <a href="http://gammaray.msfc.nasa.gov/batse/">Bursts And Transient Source Experiment (BATSE)</a> detector on board NASA’s <a href="http://en.wikipedia.org/wiki/Compton_Gamma_Ray_Observatory">Compton Gamma Ray Observatory</a> satellite was designed to test this theory. BATSE was launched on April 5, 1991 and detected GRBs at a rate of one a day. </p>
<p>BATSE could locate the position of a burst to within a few <a href="http://en.wikipedia.org/wiki/Degree_(angle)">degrees</a>, which allowed the first sky maps to be produced. What the sky maps revealed was stunning: GRBs appeared to be distributed across the entire sky, not from our galaxy at all.</p>
<p>The BATSE data indicated that GRBs occur in the very distant cosmos, billions of light years from Earth. This discovery produced a huge dilemma: they were too bright to be explained by even the most energetic processes in the universe. The energy budget required to see a GRB from Earth was just too big.</p>
<p>The only explanation for this controversial finding was the idea that energy from a GRB is focused into a beam, similar to a torch or spotlight. This would reduce the total energy required to see a GRB from Earth. </p>
<p>Could this mean that aliens had constructed a massive reflector billions of years in the past, to focus energy from a new-born black hole on Earth, with the message: “Hey, we are here”? (Actually the message should be “hey, we were here billions of years ago”, because it has taken that long for the radiation from the GRB to reach Earth!)</p>
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<p>The answer is arguably even more exotic. The generally accepted model for GRB emission is an explosion from which the particles are ejected in jets. This is analogous to a high-power jet of water emerging from a hose under pressure. </p>
<p>But for GRBs, the water is replaced with atoms stripped naked, travelling near the <a href="http://www.speed-light.info/">speed of light</a>, and focused by a rapidly rotating and highly magnetised collapsing star. <a href="http://en.wikipedia.org/wiki/Introduction_to_special_relativity">Einstein’s special relativity</a> predicts that charged particles in the jet will emit radiation that is <a href="http://en.wikipedia.org/wiki/Relativistic_beaming">“relativistically beamed”</a> in the direction of the jet.</p>
<p>This “beaming” model helps explain the too-much-energy problem, but it implies there are many more GRBs than originally thought. That is, there are bound to many GRBs we don’t see because their beams aren’t pointed at Earth.</p>
<p>One problem with this “beaming” (or <a href="http://ned.ipac.caltech.edu/level5/March04/Piran2/frames.html">“fireball”</a>) theory is that it says nothing about the actual process that creates the bursts of energy in the first place. </p>
<p>But there is growing evidence that (some) GRBs are related to a special type of rapidly spinning collapsing star that either forms a <a href="http://imagine.gsfc.nasa.gov/docs/science/know_l2/black_holes.html">black hole</a> or a neutron star. Other GRBs are likely powered by colliding or merging neutron stars.</p>
<p>It is extraordinary to imagine that most of the GRBs we have detected (thousands so far) are the cosmic signatures of black holes being created billions of years in our past.</p>
<p>So what if a GRB happened in our galaxy and one of the beams happened to be pointing at Earth?</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/7579/original/r54sb5f5-1329087778.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/7579/original/r54sb5f5-1329087778.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/7579/original/r54sb5f5-1329087778.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=419&fit=crop&dpr=1 600w, https://images.theconversation.com/files/7579/original/r54sb5f5-1329087778.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=419&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/7579/original/r54sb5f5-1329087778.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=419&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/7579/original/r54sb5f5-1329087778.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=526&fit=crop&dpr=1 754w, https://images.theconversation.com/files/7579/original/r54sb5f5-1329087778.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=526&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/7579/original/r54sb5f5-1329087778.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=526&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Artist’s conception of a massive star collapsing to form a black hole, creating a gamma ray burst in the process.</span>
<span class="attribution"><span class="source">National Science Foundation (US)</span></span>
</figcaption>
</figure>
<p>Quite simply, the effects could be devastating for life. According to <a href="http://xxx.lanl.gov/abs/0903.4710">a paper</a> written by US astronomer <a href="http://www.washburn.edu/faculty/bthomas/">Brian C. Thomas</a>, the ozone layer would be severely depleted, resulting in catastrophic exposure to UV radiation that would cause significant DNA mutations.</p>
<p>Thomas states in his paper: “In multicellular organisms, the effects may cause developmental delay and abnormalities, altered tissue composition and cancer.” </p>
<p>It’s possible that GRBs in the distant past influenced the evolution of life on Earth by creating an explosion of genetic mutations, or even mass extinctions.</p>
<p>Given new GRBs are being detected almost daily, you’d be forgiven for thinking the chance of a nearby GRB is high. But if you take into account the enormous distances from Earth that most GRBs occur, the probability for one occurring in our galaxy is actually very small. </p>
<p>Of course, over geological time (billions of years) it’s plausible that a GRB beam did irradiate Earth in the past and could do so again. </p>
<p>Introducing <a href="http://en.wikipedia.org/wiki/WR_104">WR 104</a> – a massive star located within our galaxy some 8,000 light years from Earth. Sydney University astronomer <a href="http://www.physics.usyd.edu.au/%7Egekko/">Peter Tuthill</a> discovered that <a href="http://www.physics.usyd.edu.au/%7Egekko/pinwheel.html">this star is near the end of its life</a>, and will one day explode as a <a href="http://en.wikipedia.org/wiki/Hypernova">“hypernova”</a>, then collapse to form a black hole. </p>
<p><a href="http://www.physics.usyd.edu.au/%7Egekko/pinwheel.html">WR 104 is rotating with its poles roughly in line with Earth</a>, so if the collapsing star happened to produce a GRB, its jet and beam would be pointing toward Earth. </p>
<p>When will this occur? Maybe tomorrow, maybe thousands of years from now. I just hope it doesn’t happen at the <a href="http://en.wikipedia.org/wiki/2012_phenomenon">end of the Mayan calendar</a> on December 21 this year, because there will be a lot of people saying “I told you so!”</p><img src="https://counter.theconversation.com/content/5291/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Coward receives funding from the Australian Research Council.</span></em></p>Ever since they were discovered accidentally in the 1960s, gamma ray bursts (GRBs) have continued to amaze and puzzle astronomers worldwide. In nearly 50 years of research there seem to have been more…David Coward, ARC Future Fellow; Ass Prof., The University of Western AustraliaLicensed as Creative Commons – attribution, no derivatives.