On the morning of October 9, multiple space-based detectors picked up a powerful gamma-ray burst (GRB) passing through our solar system, sending astronomers around the world scrambling to train their telescopes on that part of the sky to collect vital data on the event and its afterglow. Dubbed GRB 221009A, astronomers say the gamma-ray burst is the most powerful yet recorded and likely could be the "birth cry" of a new black hole. The event was promptly published in the Astronomer's Telegram, and observations are still ongoing.
“In our research group, we’ve been referring to this burst as the ‘BOAT,’ or Brightest Of All Time, because when you look at the thousands of bursts gamma-ray telescopes have been detecting since the 1990s, this one stands apart,” said Jillian Rastinejad, a graduate student at Northwestern University. Rastinejad led one of two independent teams using the Gemini South telescope in Chile to study the event's afterglow.
“This burst is much closer than typical GRBs, which is exciting because it allows us to detect many details that otherwise would be too faint to see,” said Roberta Pillera, a graduate student at the Polytechnic University of Bari, Italy, and member of the Fermi Large Area Telescope (LAT) Collaboration. “But it’s also among the most energetic and luminous bursts ever seen regardless of distance, making it doubly exciting.”
Gamma-ray bursts are extremely high-energy explosions in distant galaxies lasting between mere milliseconds to several hours. The first gamma-ray bursts were observed in the late 1960s, thanks to the launching of the Vela satellites by the US. They were meant to detect telltale gamma-ray signatures of nuclear weapons tests in the wake of the 1963 Nuclear Test Ban Treaty with the Soviet Union. The US feared that the Soviets were conducting secret nuclear tests, in violation of the treaty. In July 1967, two of those satellites picked up a flash of gamma radiation that was clearly not the signature of a nuclear weapons test.
That data was filed away, but later Vela satellites with improved instruments recorded several more gamma-ray bursts. A team at Los Alamos National Laboratory analyzed when each burst was detected by different satellites to estimate the sky position of 16 such bursts. And they determined that the bursts weren't from Earth or our Solar System, publishing their conclusions in a 1973 paper in Astrophysical Journal.
There are two classes of gamma-ray bursts. Most (70 percent) are long bursts lasting more than two seconds, often with a bright afterglow. These are usually linked to galaxies with rapid star formation. Astronomers think that long bursts are tied to the deaths of massive stars collapsing to form a neutron star or black hole (or, alternatively, a newly formed magnetar). The baby black hole would produce jets of highly energetic particles moving near the speed of light, powerful enough to pierce through the remains of the progenitor star, emitting X-rays and gamma rays.
Those gamma-ray bursts lasting less than two seconds (about 30 percent) are deemed short bursts, usually emitting from regions with very little star formation. Astronomers think these gamma-ray bursts are the result of mergers between two neutron stars, or a neutron star merging with a black hole, comprising a "kilonova."
That hypothesis was confirmed in 2017, when the LIGO collaboration picked up the gravitational wave signal of two neutron stars merging, accompanied by the powerful gamma-ray bursts associated with a kilonova. Earlier this year, astrophysicists spotted mysterious X-rays they believed could be the very first detection of a kilonova "afterglow" from that same merger. (Alternatively, it could be the first observation of matter falling into the black hole that formed after the merger.)
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