Spotting the brightest gamma-ray burst ever recorded

UNIVERSITY PARK, Pa. — When the Neil Gehrels Swift Observatory sent an automatic alert to her phone on Oct. 9, 2022, Penn State research technologist Maia Williams had no idea that the space telescope had just detected the brightest gamma-ray burst ever recorded. Or that, although she was new to the Swift team, she would be responsible for sharing the news of this cosmic explosion.

Williams had only joined Swift’s Missions Operation Center — which is located at Penn State — a few months prior, shortly after graduating from Bowling Green State University. The Swift Observatory is dedicated to studying gamma-ray bursts — the most powerful types of explosions in our universe, which can result from the death of a massive star. As the dying star collapses into a black hole or neutron star, it sends gamma rays, X-rays, and other particles into space, which can be detected by observatories like Swift as they approach Earth. Swift was the first observatory to report the burst, which was named GRB 221009A.

“This one was weird, because at first it didn’t seem like a gamma-ray burst,” said Williams. “It was uncommonly bright, and its location seemed unusual for a gamma-ray burst. This was a little disappointing for me because it would have been my first gamma-ray burst that I had been assigned to follow up on. But we soon learned that it had also been detected by NASA’s Fermi Gamma-ray Space Telescope, and it was one of the brightest bursts they had seen. And then we heard from other observatories as well, so we knew this was a unique and incredibly bright gamma-ray burst.”

In fact, the burst was so bright that it has been dubbed the "BOAT" — the “brightest of all time,” or at least since astronomers started recording these events. Williams and colleagues found that the afterglow of the explosion, which gradually fades over time as the particles expand into space, was more than 10 times brighter than that of any previous gamma-ray bursts observed by Swift.

“The burst was intrinsically energetic, but it was also located relatively close to the Earth, as far as gamma-ray bursts go,” said Williams. “So some of its brightness can be attributed to its proximity.”

Astronomers believe the source of the gamma-ray burst is about 1.9 billion light years away, making it one of the closest-known gamma-ray bursts. Swift detects about 100 gamma-ray bursts a year, mostly at a distance around 10 billion light years away. Because their light has further to travel, the light of most of these bursts appears dimmer when it reaches Earth.

“Based on our simulations, a gamma-ray burst as energetic and as close as this one is likely to occur less than once every 1,000 years, so this really is a remarkable event that we’re unlikely to see again in our lifetimes,” said Williams. “When we first detected this burst, I had only been at Swift for a few months, so I didn’t realize how extraordinary this was.”

On Oct. 9, Williams had been assigned as that day’s “burst advocate,” responsible for following up on the initial observation of any burst detected that day, gathering data from other observatories about the burst, and regularly reporting on the burst to the Swift team. Williams had received some training about the role and was expected to learn on the job once a burst was detected. But, she said, she didn’t expect her first burst to be so noteworthy.

“It was pretty overwhelming at times, but I definitely learned a lot!” she said. 

After the initial observation, Swift dedicated about 5,000 seconds of observing time to the gamma-ray burst each day through the end of November. Because the burst was so unusually bright, they continued to observe the burst every other day through December, when the Earth moved to a position where the burst was blocked by the sun. Several other observatories, both in space and on the Earth’s surface, also observed the burst, giving Williams plenty of data to report on. She also led efforts to write a scientific paper reporting the burst, which involved working with collaborators and synthesizing data from Swift as well as the MAXI (Monitor of All-sky X-ray Image) and NICER (Neutron Star Interior Composition Explorer Mission) missions, whose instruments are located on the International Space Station.

“There was a lot of organizing meetings, coordinating across time zones, and trying to make sure all the work got done,” she said. “And there was a lot of editing to do, because everyone writes in a different style, and I wanted to make sure that everything in the paper felt consistent. I’m really glad I had a lot of help, especially from the other members of the Science Operations Team at Swift.”

The paper published in a special focus issue of the Astrophysical Journal Letters. Other Penn State authors at the Swift Missions Operations Center include Jamie Kennea, research professor of astronomy and astrophysics and leader of the Swift Science Operations Team; Simone Dichiara, assistant research professor of astronomy and astrophysics; and Michael Siegel, research professor of astronomy and astrophysics.

The BOAT gamma-ray burst re-appeared from behind the sun in February, and Swift continues to observe the afterglow, albeit not quite as often (about 10,000 seconds once a week). Studying the burst, and others like it, could help astronomers understand how stars collapse, how black holes are born, and the conditions in distant galaxies. 

“We hope that continuing to observe this gamma-ray burst will help us explain why it was so bright,” said Williams. “This experience and the support I received from the Swift team has definitely prepared me to serve as burst advocate for future gamma-ray bursts that we might detect. I’m excited to see what future discoveries we make with Swift at the Missions Operation Center at Penn State.”