It’s the BOAT: Astronomers observe ‘brightest-ever’ gamma-ray burst

On the morning of October 9, several space-based detectors picked up a powerful gamma-ray burst (GRB) that traversed our solar system, prompting astronomers around the world to turn their telescopes to this part of the sky to collect important data about the event and its afterglow. Astronomers say the gamma-ray burst, dubbed GRB 221009A, is the most powerful yet recorded and could likely be the “birth cry” of a new black hole. The event was immediately published in the Astronomer’s Telegram and observations are still ongoing.

“In our research group, we refer to this outburst as the ‘BOOT,’ or brightest of all time, because when you look at the thousands of outbursts that gamma-ray telescopes have detected since the 1990s, this one stands out,” 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 explosion is much closer than typical GRBs, which is exciting because it allows us to see a lot of detail that would otherwise be too faint to see,” said Roberta Pillera, a PhD student at the Polytechnic University of Bari, Italy, and a member of the Fermi Large Area Telescope (LAT) Collaboration. “But it’s also among the most energetic and luminous explosions ever seen, regardless of distance, which makes it doubly exciting.”

Gamma-ray bursts are extremely energetic explosions in distant galaxies that last anywhere from a few milliseconds to several hours. The first gamma-ray bursts were observed in the late 1960s thanks to the US launch of the Vela satellites. They should detect telltale gamma-ray signatures from nuclear weapons tests under the 1963 Nuclear Test Ban Treaty with the Soviet Union. The US feared that the Soviets would conduct secret nuclear tests in violation of the treaty. In July 1967, two of these satellites picked up a burst of gamma rays that was clearly not a nuclear weapons test signature.

This data was discarded, but later, Vela satellites with improved instruments recorded several more gamma-ray bursts. A team from Los Alamos National Laboratory analyzed when each burst was spotted by different satellites to estimate the sky location of 16 such bursts. And they determined that the outbursts were not from Earth or our solar system, and published their conclusions in a 1973 article in the 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 associated with fast star forming galaxies. Astronomers believe long bursts are related to the deaths of massive stars, which collapse and form a neutron star or black hole (or alternatively, a newly formed magnetar). The tiny black hole would create jets of high-energy particles traveling at nearly the speed of light, powerful enough to penetrate the remains of the progenitor star and emit X-rays and gamma rays.

These gamma-ray bursts, which last less than two seconds (about 30 percent), are thought to be brief bursts typically emitted from regions of very low star formation. Astronomers believe that these gamma-ray bursts are the result of the merger of two neutron stars or the merger of a neutron star with a black hole that is a “kilonova.”

This hypothesis was confirmed in 2017 when the LIGO collaboration picked up the gravitational-wave signal from two merging neutron stars accompanied by the strong gamma-ray bursts associated with a kilonova. Earlier this year, astrophysicists discovered mysterious X-rays that they believe could be the first-ever detection of a kilonova “afterglow” from the same fusion. (Alternatively, it could be the first observation of matter falling into the black hole that formed after the merger.)