The first instrument to detect a gamma ray burst (GRB) was watching for the end of the world. What it ended up seeing was quite possibly the end of someone else’s.
In the 1960s, during the height of the Cold War, the US military deployed a group of satellites called Vela to monitor the planet and our vicinity in outer space for the characteristic flash of high-energy radiation – gamma rays and X-rays – that would give away the location of secret nuclear weapon tests, even if they were being carried out behind the Moon.
The first events Vela observed, however, looked nothing like the kind of flash a nuclear bomb would produce, and seemed to be coming from deep space. For years afterward, there ensued a long debate about their nature and origin: where could these spectacular bursts of radiation be coming from? Some argued they were extraordinarily bright explosions outside our Galaxy. Others thought nothing could possibly be that bright – perhaps they were some kind of ultra-powerful solar flare from stars closer to us.
The debate about their distance was eventually won through a clever argument that had little to do with the bursts themselves. Observers had already noticed that the bursts were appearing all over the sky, with no extra concentration in any particular direction. Polish astronomer Bohdan Paczyński pointed out that left only two options. One: they are very close by, in our immediate vicinity, presumably arising from very nearby stars, or two: they’re so far away that they must be in very distant galaxies.
In both cases, they’d appear everywhere, either because the clump of stars around us is too small a piece of the Milky Way for any of the Galaxy’s structure to affect the distribution, or because on the largest scales in the cosmos, there’s no structure big enough to stand out. It would be like if you saw a very fuzzy, low-resolution image of little yellow dots on a green background, and you didn’t know if you were looking at a close-up photo of the petals of a small bunch of yellow flowers, or an aerial shot of yellow bushes scattered across a field. If it were anything in the middle, you’d see the shape of the bush, or a clump of bushes.
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But from close enough in or far enough away, the yellow is scattered about pretty much everywhere. In the case of GRBs, we had good reason to think they weren’t that nearby, which left cosmological distances as the only possibility.
GRBs are now thought to have two origins. A ‘long’ GRB occurs when a rapidly rotating massive star collapses on itself, creates a supernova, and in the process causes energetic jets of radiation to shoot out through the poles of the stellar remnant. A ‘short’ GRB is caused by two neutron stars, themselves remnants of dead stars, colliding. Long GRBs made the news in a spectacular way in early October when what may have been the brightest burst ever recorded lit up the cosmos with so much power that it temporarily blinded several satellites and altered radio wave transmission by disturbing the Earth’s ionosphere.
Officially dubbed GRB221009A, it also created a striking set of concentric rings in X-ray images, due to the way the light from the explosion bounced off layers of cosmic dust. Preliminary analyses of the burst suggest the star was so massive that it became a black hole after the explosion, and follow-up observations have found an optical-light afterglow that appears to be associated with the supernova.
We don’t know if the star was orbited by any planets before it went off. If it did have any planets, they would have been destroyed by the supernova, if not the burst itself. But a GRB can cause devastation far beyond its own solar system. Some estimates suggest that a planet in the path of the beam could be vaporised even 200 light years away, and impacts on an atmosphere could be felt even farther. There have been some hints that GRBs millions of years ago could have changed the chemistry of our own atmosphere enough to set off mass extinctions.
Should we be worried? Probably not. Of the 1,700 or so recorded GRBs, the closest was a billion light-years away, far too distant to be an issue for us. And as far as we can tell, no nearby stars are massive enough to be a threat, and we don’t have reason to believe there are any close neutron star pairs in danger of colliding soon, either.
As astronomers, we’re grateful for GRBs: they help us learn exactly how and why stars explode in such a spectacular way. They also give us a useful probe of the cosmic material between us and them, by shining a very bright backlight on it.
Some theorists have even suggested that GRB221009A’s most energetic photons might only have reached us with the help of exotic hypothetical particles called axions. As we keep watching for new GRBs, we’ll have more data to help settle some of these questions, and fortunately, we don’t have to get a close-up look.
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