In April 1998, light from the explosive death of a massive star 120 million light-years away irradiated Earth.
This was SN 1998bw, the very first collapsar supernova to be observed, emitted by a massive stellar core undergoing gravitational collapse, and the very first supernova to be associated with a gamma-ray burst. Now, astronomers have detected a new gamma-ray burst, and it's the shortest we've ever detected in association with a collapsar.
It's called GRB 200826A, and it could change how we understand these incredibly energetic events.
"Our panchromatic follow-up data confirm a collapsar origin," wrote a team of researchers led by astronomer Tomás Ahumada of the University of Maryland.
"GRB 200826A is the shortest long-soft gamma-ray burst found with an associated collapsar; it appears to sit on the brink between a successful and a failed collapsar. Our discovery is consistent with the hypothesis that most collapsars fail to produce ultra-relativistic jets."
Collapsars are also known as hypernovae, and they're thought to be the result of an extreme core-collapse star death. They're some of the most energetic supernovae in the Universe, occurring when the core of a star greater than about 30 solar masses collapses down to form a rapidly spinning black hole.
GRB 200826A, detected in August 2020, was thought to be a different kind of gamma-ray burst, known as a short gamma-ray burst - such as those emitted by a compact binary merger, like two neutron stars.
It might have stayed that way, too, if Ahumada and his colleagues hadn't discovered another transient, and rapidly fading, bloom of light: ZTF20abwysqy. This, they have now confirmed, is the afterglow of GRB 200826A - and its emission profile is consistent not with a binary merger, but a supernova.
A separate team, led by astrophysicist Binbin Zhang of Nanjing University in China, independently arrived at the same conclusion with their analysis of GRB 200826A.
"Characterized by a sharp pulse, this burst shows a duration of 1 second and no evidence of an underlying longer-duration event. Its other observational properties such as its spectral behaviors, total energy and host galaxy offset are, however, inconsistent with those of other short GRBs believed to originate from binary neutron star mergers," Zhang's team wrote in their paper.
"Rather, these properties resemble those of long GRBs. This burst confirms the existence of short-duration GRBs with stellar core-collapse origin."
This association could change our understanding of these extreme, energetic events. Gamma-ray bursts are thought to be associated with relativistic jets - that is, jets of plasma blasting out at a significant percentage of the speed of light in a vacuum - launched by the newly formed black hole as it accretes material.
According to Ahumada and colleagues, the short duration of the burst indicates that the jet may not have formed, or been unable to blast free of the material around the collapsed star.In addition, the discovery suggests that many events classified as short gamma-ray bursts may actually be misclassified long gamma-ray bursts - that is, we thought we were looking at neutron star mergers, but they're actually hypernovae with hindered jets.
And if that's the case, then our perception of the Universe may have just shifted a little - because it would mean that failure to launch jets may actually be quite common for collapsars - and that collapsars aren't as rare as we thought.
This would be pretty interesting, because collapsars are thought by some to be one of the major sources of heavy elements in the Universe. The origin of these elements is something of a puzzle, which a boosted collapsar rate might help solve.
And the research underscores just how important it is to keep our eyes on the sky.
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