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Few cosmic explosions have attracted as much attention from space scientists as the one recorded on October 22 last year and rightly called the brightest of all time (BOAT). The event, produced by the collapse of a very massive star and subsequent birth of a black hole, was seen as an immensely bright burst of gamma rays followed by a flare of light that slowly fades across frequencies.
Since simultaneously detecting the BOAT signal on their giant telescopes, astrophysicists around the world have scrambling to explain the brightness of the gamma-ray burst (GRB) and the curiously slow fade of its afterglow.
Now an international team including Dr Hendrik Van Eerten of the Physics Department of the University of Bath in the UK has come up with an explanation: the initial explosion (known as GRB 221009A) was angled directly at Earth and also dragged an unusually large amount of stellar material in its wake.
The team’s findings were published June 7 in the journal The progress of science. Dr. Brendan O’Connor, a recent graduate doctoral student at the University of Maryland and George Washington University in Washington, DC, is the study’s lead author.
Dr Van Eerten, who co-led the theoretical analysis of the afterglow, said: ‘Other researchers working on this puzzle have also come to the conclusion that the jet was aimed directly at us, just like a garden hose. angled to spray straight at you and that certainly goes some way to explaining why it was viewed so brilliantly.”
But what remained a puzzle was that the edges of the jet could not be seen at all.
“The slow fading of the afterglow is not characteristic of a narrow jet of gas, and knowing this made us suspect that there was an ulterior reason for the intensity of the explosion, and our mathematical models confirmed it.
“Our work clearly shows that the GRB had a unique structure, with observations gradually revealing a narrow jet embedded within a larger gas outflow where an isolated jet would normally be expected.”
So what made this GRB wider than normal? Researchers have a theory. As explained by Dr. Van Eerten, “GRB jets have to pass through the collapsing star in which they form, and what we think made the difference here was the amount of mixing that occurred between the stellar material and the jet, such that the gas heated by the impact continued to appear in our line of sight to the point where any characteristic features of the jet would have been lost in the overall afterglow emission.”
He added: ‘Our model helps not only to understand the BOAT, but also previous luminosity record holders who had puzzled astronomers about their lack of jet signature. These GRBs, like other GRBs, must be aimed directly at us when they occur, as it would not be physical for so much energy to be thrown out in all directions at once.
“There appears to be an exceptional class of events that are both extreme and succeed in masking the directed nature of their gas flow. Future studies of the magnetic fields that launch the jet and the massive stars that host them should help reveal why these GRBs are So rare.”
Dr. O’Connor said: “The exceptionally long GRB 221009A is the brightest GRB on record and its afterglow is breaking all records at all wavelengths. Why is this flash so bright and even close (cosmically speaking: it occurred as close as 2.4 billion light-years from Earth), we think this is a once-in-a-thousand-year opportunity to address some of the fundamental questions surrounding these explosions, from the formation of black holes to testing dark matter models”.
Brendan O’Connor et al, A structured jet explains the extreme GRB 221009A, The progress of science (2023). DOI: 10.1126/sciadv.adi1405. www.science.org/doi/10.1126/sciadv.adi1405
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The progress of science
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