The gamma-ray burst (GRB) known as the Brightest Of All Time (BOAT) had an emission line in its afterglow, the first time one has been detected with confidence in any GRB. The astronomers who discovered the line suspect it is the product of matter and antimatter particles annihilating each other in the aftermath of the explosion while hurtling toward us at close to the speed of light.
For most natural phenomena, the most dramatic example recorded is not all that far ahead of the second or third – think earthquakes, volcanoes, or colliding black holes. When it comes to GRBs, however, that pattern breaks down. The BOAT (officially GRB 221009A) earned its name by being 70 times brighter than the next brightest we have seen. Based on the pattern of other GRBs, we should expect to see something like this once every thousand (some say ten thousand) years, not in the first few decades after we had the instruments to observe it. It’s as if someone ran the marathon in under an hour, while every other entrant was slower than the existing world record.
Unsurprisingly, astronomers are keen to extract every scrap of information they can about this unparalleled event, but are hindered by the fact its initial brightness saturated gamma ray cameras. Now, an examination of the radiation shortly afterward has revealed something astronomers have sought in other GRBs: an emission line.
The release of electromagnetic energy at a particular wavelength creates an emission line within a broader spectrum. These lines are usually produced by electrons transitioning from excited states, releasing an amount of energy specific to that element. Emission, and corresponding absorption lines, have taught us most of what we know about the nature of stars – but the only previous potential detections in GRBs were so weak they were thought likely to be statistical fluctuations.
“A few minutes after the BOAT erupted, Fermi’s Gamma-ray Burst Monitor recorded an unusual energy peak that caught our attention,” said Dr Maria Edvige Ravasio, of Radboud University in Nijmegen, in a statement. “When I first saw that signal, it gave me goosebumps. Our analysis since then shows it to be the first high-confidence emission line ever seen in 50 years of studying GRBs.”
As co-author Dr Om Sharan Salafia at INAF-Brera Observatory added; “We’ve determined that the odds this feature is just a noise fluctuation are less than one chance in half a billion.” Then again, before the BOAT occurred, the chances of seeing something so powerful would have been rated not much higher, so perhaps we shouldn’t dismiss such odds.
By saturating instruments such as FERMI, the BOAT prevented us from learning much about the crucial first seconds. The peak brightness has had to be reconstructed from what we could see once intensity dropped to manageable levels. The line appeared almost five minutes after the explosion started and lasted for at least 40 seconds.
GRBs come in two sorts, thought to have different sources, although astronomers can’t yet rule out the possibility the BOAT represents the only example we have seen of a mysterious third class. So far, however, they are treating it as a more powerful version of what we know, in which case it would be the consequence of a massive star collapsing to form a black hole. As well as the supernova explosion such an event creates, immensely powerful jets are produced. When one is pointed at Earth, we are exposed to a burst of gamma rays lasting a few hundred seconds at most.
The emission line started at 0.00099 Angstroms or 12.56 Million electron volts and was last detected at half that value. Blue light is 3 eV, and some of the gamma rays used in radiotherapy have energies of 1.2 MeV.
Energy like that certainly doesn’t come from excited electrons returning to their ground state, so the team who made the discovery had to find a new explanation. They think the cause is the annihilation of electrons and positrons (the anti-matter equivalent of an electron).
“When an electron and a positron collide, they annihilate, producing a pair of gamma rays with an energy of 0.511 MeV,” said Professor Gor Oganesyan of the Gran Sasso Science Institute. “Because we’re looking into the jet, where matter is moving at near light speed, this emission becomes greatly blueshifted and pushed toward much higher energies.”
A blueshift this size would require the particles to have been accelerated to 99.9 percent of the speed of light, but that seems unremarkable in the context of everything else about the BOAT. A tiny slowdown would reduce the blue shift and explain why the energy fell during the detection period.
“After decades of studying these incredible cosmic explosions, we still don’t understand the details of how these jets work,” said Elizabeth Hays of the Goddard Space Flight Center. “Finding clues like this remarkable emission line will help scientists investigate this extreme environment more deeply.”
Just imagine what we would have seen if instruments like Fermi had been built with the capacity to record the BOAT’s brightest part – something not previously thought necessary.
The findings are published in the journal Science.
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