On the morning of October 9, 2022, 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 aftermath. Dubbed GRB 221009A and deemed likely to be the "birth cry" of a new black hole, the gamma-ray burst is the most powerful yet recorded. That's why astronomers nicknamed it the BOAT, or Brightest Of All Time.
The event was promptly published in the Astronomer's Telegram, and we now have new data from follow-up observations in several new papers published in a special focus issue of the Astrophysical Journal Letters. The findings confirmed that GRB 221009A was indeed the BOAT, appearing especially bright because its narrow jet was pointing directly at Earth. “It’s probably the brightest event to hit Earth since human civilization began,” Eric Burns, an astronomer at Louisiana State University, told New Scientist. “The energy of this thing is so extreme that if you took the entire sun and you converted all of it into pure energy, it still wouldn’t match this event. There’s just nothing comparable.”
But the various analyses also yielded several surprising results that puzzle astronomers and may lead to a significant overhaul of our current models of gamma-ray bursts. For instance, a supernova should have occurred a few weeks after the initial burst, but astronomers have yet to detect one. Radio data from observations of the afterglow didn't match predictions of existing models, and astronomers detected rare extended rings of X-ray light echoes from the initial blast in distant dust clouds.
As we've reported previously, gamma-ray bursts are extremely high-energy explosions in distant galaxies lasting between mere milliseconds to several hours. 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. Last 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.)
The October 2022 gamma-ray burst falls into the long category, lasting over 300 seconds. GRB 221009A triggered detectors aboard NASA's Fermi Gamma-ray Space Telescope, the Neil Gehrels Swift Observatory, and Wind spacecraft, among others, just as gamma-ray astronomers had gathered for an annual meeting in Johannesburg, South Africa. The powerful signal came from the constellation Sagitta, traveling some 1.9 billion years to Earth.
After GRB 221009A was first detected, the Swift Observatory, among others, continued to observe the burst every day through the end of November and every other day through December, at which point the Earth's position meant our view of the burst was blocked by the Sun. (Swift resumed regular weekly observations in February.) Various observatories collected data spanning the electromagnetic spectrum—from the radio to gamma-ray regimes—to learn as much as possible about the event.
For instance, radio wave data revealed that GRB 221009A was 70 times brighter than any previously observed gamma-ray burst, so it is indeed the BOAT (thus far)—likely a one-in-10,000-year event. The burst's energy wasn't especially large for a GRB, but the jet emitting that energy was unusually narrow—and pointing directly toward Earth, making GRB 221009A seem especially bright.
But astronomers have yet to detect evidence of an associated supernova, perhaps because thick dust clouds in that part of the sky (just a few degrees above the plane of our own galaxy) are dimming any incoming light. “We cannot say conclusively that there is a supernova, which is surprising given the burst’s brightness,” said Andrew Levan, an astrophysicist at Radboud University in Nijmegen, Netherlands, who led near- and mid-infrared observations using NASA’s Webb Telescope and the Hubble Space Telescope in hopes of spotting the expected supernova. "If it’s there, it’s very faint. We plan to keep looking, but it’s possible the entire star collapsed straight into the black hole instead of exploding.”
Although GRBs are generally over in mere seconds, they leave afterglow emissions across the light spectrum that can echo for months or even years, and the follow-up observational data in various spectra gave astronomers a rare opportunity to explore the evolution of that afterglow in detail. They were surprised to find that the radio data showed the jet evolved smoothly and fairly slowly over time, contradicting existing models showing fast jumps in energy as a jet evolves.
“Twenty-five years of afterglow models that have worked very well cannot completely explain this jet,” said Kate Alexander, an astronomer at the University of Arizona in Tucson. “This [new radio component] may indicate additional structure within the jet or suggest the need to revise our models of how GRB jets interact with their surroundings.”
“A few GRBs in the past have shown a brief excess of millimeter and radio emission that is thought to be the signature of a shockwave in the jet itself, but in GRB 221009A the excess emission behaves quite differently than in these past cases,” said Yvette Cendes of the Harvard-Smithsonian Center for Astrophysics. "It is likely that we have discovered a completely new mechanism to produce excess millimeter and radio waves. It is possible that the visible and X-ray light are produced by one portion of the jet, while the early millimeter and radio waves are produced by a different component.”
Other astronomers turned their attention to distant dust clouds in our Milky Way galaxy and found that 21 of these clouds had scattered X-rays from the burst, producing a series of light echoes in the form of X-ray rings. Because distance, the size of dust grains, and the energies of the X-rays all factor into how the clouds scatter X-rays, astronomers could use the ring data to reconstruct the X-ray emission to pinpoint where the dust clouds were located. The X-ray ring data also revealed a small degree of polarization in the afterglow—more confirmation that the jet was aimed almost directly at Earth.
