The gamma-ray burst GRB 221009A was so far off the charts for these events that we would only expect to see something this bright once every thousand years. Naturally, astronomers expected some phenomenal event to be responsible – but instead, they found what looks like a perfectly ordinary supernova, with few clues as to why it would produce something so dazzling.
Most sorts of events follow certain patterns in distribution and size. The most powerful earthquake or volcanic eruption is unlikely to be all that different from the second and third example. All of which makes GRB 221009A an ongoing puzzle to astronomers. It was so bright it saturated satellite gamma-ray detectors, despite being 2.4 billion light years away, preventing us from measuring it directly. However, by studying its afterglow, astronomers have concluded it was 70 times as bright as the current second place.
Astronomers nicknamed GRB 221009A the Brightest Of All Time (B.O.A.T) and estimated we should see something like this once every thousand years, which makes it rather surprising it turned up in the few decades satellite gamma-ray detectors have existed.
The so-called afterglow of the GRB was like the headlights of a car coming straight at you, preventing you from seeing the car itself. So, we had to wait for it to fade significantly to give us a chance of seeing the supernova
Dr Peter Blanchard
Long gamma-ray bursts like this one are thought to be associated with the birth of black holes, usually from supernovae in stars with more than 25 solar masses. We might guess this was a particularly enormous supernova, producing an exceptional black hole. If so, it would make sense for the event to have initiated the r-process, thought to be responsible for elements such as platinum and gold.
A large team has used the JWST to study the afterglow of GRB 221009A’s supernova for signs of something out of the box that could be responsible. They found the afterglow of the supernova responsible, but the spectrum was neither particularly bright nor rich in precious metals. Midas this was not.
“When we confirmed that the GRB was generated by the collapse of a massive star, that gave us the opportunity to test a hypothesis for how some of the heaviest elements in the universe are formed,” said Dr Peter Blanchard of Northwestern University in a statement.
Instead of rushing in straight away, when the GRB’s aftereffects would have overshadowed the accompanying supernova, they chose patience. Initially, Blanchard said; “The so-called afterglow of the GRB was like the headlights of a car coming straight at you, preventing you from seeing the car itself. So, we had to wait for it to fade significantly to give us a chance of seeing the supernova.”
Almost six months later the time was judged to be right. Using the JWST Blanchard and coauthors spotted familiar signatures of elements like oxygen and nickel that are hallmarks of supernovae, but the glow was not proportionally as bright as the co-occurring GRB.
Moreover; “We did not see signatures of these heavy elements, suggesting that extremely energetic GRBs like the B.O.A.T. do not produce these elements. That doesn’t mean that all GRBs do not produce them, but it’s a key piece of information as we continue to understand where these heavy elements come from. Future observations with JWST will determine if the B.O.A.T.’s ‘normal’ cousins produce these elements.”
The findings leave at least two big mysteries: why the discrepancy between GRB and supernova brightness, and where do heavy elements come from? We know the r-process occurs in kilonovae, where two neutron stars merge, but these are such rare events there are doubts about whether they can account for all the heavy elements we see.
“There is likely another source,” Blanchard said. “It takes a very long time for binary neutron stars to merge. Two stars in a binary system first have to explode to leave behind neutron stars. Then, it can take billions and billions of years for the two neutron stars to slowly get closer and closer and finally merge. But observations of very old stars indicate that parts of the universe were enriched with heavy metals before most binary neutron stars would have had time to merge. That’s pointing us to an alternative channel.”
If the brightest GRB of all time isn’t that other source, what is? Answer that (correctly) and you’ll write your name in astronomical history. Just as important could be explaining how an ordinary supernova and an epic GRB come to form the astronomical version of Notting Hill.
The B.O.A.T was so bright it saturated satellite detectors. This image was taken by SWIFT’s X-Ray Telescope an hour after the GRB, which only lasted a few minutes.
Part of the answer may be that the gamma rays from the B.O.A.T appear to have been unusually focused. The stars that trigger long GRBs are thought to be rotating particularly fast prior to their explosions, which leads them to launch jets of material at close to the speed of light when their big moment comes. The narrower the jets, the more focused the beam of gamma rays becomes, making it less likely a random galaxy like our own will be in the beam – but making it much brighter if it is. Why the B.O.A.T’s jets were so narrow isn’t known, but at least it makes the issue a little more comprehensible.
Another possible part of the answer may lie in the B.O.A.T’s host galaxy. The JWST revealed this as being an extreme starburst galaxy, where new stars are forming at an exceptional rate. More new stars mean a higher chance of supernovae, but maybe it also affects the ones that occur in some unknown way. The galaxy is also almost pure hydrogen and helium, with about an eighth the concentration of metals of the Sun, the lowest yet seen in a host of a gamma-ray burst supernova combination. That means the star formation must be very new, as previous generations of stars would have raised the metal content.
The authors don’t yet know how these features could have contributed to this exceptional event, but they’re probably relevant somehow.
The study is published in the journal Nature Astronomy.
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