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 theories trying to explain GRBs then there have been actual GRBs.
So just what are GRBs? Where do they come from? And what would happen if one occurred in our cosmic backyard?
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 Partial Nuclear Test Ban Treaty.
The US Vela satellites were launched in 1962 to enforce this treaty, by monitoring space for the flashes of gamma rays emitted from nuclear explosions.
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?
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.
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.
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 neutron stars in our galaxy, and even communications from aliens.
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.
The Bursts And Transient Source Experiment (BATSE) detector on board NASA’s Compton Gamma Ray Observatory satellite was designed to test this theory. BATSE was launched on April 5, 1991 and detected GRBs at a rate of one a day.
BATSE could locate the position of a burst to within a few degrees, 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.
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.
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.
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!)
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.
But for GRBs, the water is replaced with atoms stripped naked, travelling near the speed of light, and focused by a rapidly rotating and highly magnetised collapsing star. Einstein’s special relativity predicts that charged particles in the jet will emit radiation that is “relativistically beamed” in the direction of the jet.
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.
One problem with this “beaming” (or “fireball”) theory is that it says nothing about the actual process that creates the bursts of energy in the first place.
But there is growing evidence that (some) GRBs are related to a special type of rapidly spinning collapsing star that either forms a black hole or a neutron star. Other GRBs are likely powered by colliding or merging neutron stars.
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.
So what if a GRB happened in our galaxy and one of the beams happened to be pointing at Earth?
Quite simply, the effects could be devastating for life. According to a paper written by US astronomer Brian C. Thomas, the ozone layer would be severely depleted, resulting in catastrophic exposure to UV radiation that would cause significant DNA mutations.
Thomas states in his paper: “In multicellular organisms, the effects may cause developmental delay and abnormalities, altered tissue composition and cancer.”
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.
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.
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.
Introducing WR 104 – a massive star located within our galaxy some 8,000 light years from Earth. Sydney University astronomer Peter Tuthill discovered that this star is near the end of its life, and will one day explode as a “hypernova”, then collapse to form a black hole.
WR 104 is rotating with its poles roughly in line with Earth, so if the collapsing star happened to produce a GRB, its jet and beam would be pointing toward Earth.
When will this occur? Maybe tomorrow, maybe thousands of years from now. I just hope it doesn’t happen at the end of the Mayan calendar on December 21 this year, because there will be a lot of people saying “I told you so!”