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Wednesday, February 21, 2024

The ‘Brightest of All Time’ Gamma-Ray Burst Sparks a Supernova Hunt

On the morning of October 9, astronomers’ inboxes pinged with a relatively modest alert: NASA’s Swift Observatory had just detected a fresh burst of energy, assumed to be coming from somewhere within our own galaxy. But six hours later—when scientists realized an instrument on the Fermi Space Telescope had also flagged the event—another more pressing email arrived. “We believe that this source is now likely a gamma-ray burst,” it read. “This would suggest a highly energetic outburst, and therefore we strongly encourage follow-up.” In other words, this was a career-making chance to catch a rare celestial event in real time.

Astronomers around the world sprang into action. They were eager to point their telescopes at this powerful, jetted explosion of the most energetic photons in our universe. “And by jetted, I mean like a firehose of emission,” says Wen-fai Fong, an astrophysicist at Northwestern University. Blasts like this are thought to be caused by the supernovae of giant stars, destructive collapses that give birth to black holes. The burst, dubbed GRB 221009A, went off about 2 billion light years away in the Sagitta constellation—one of the closest and most energetic ever observed—and it’s likely that one of the jets was fortuitously pointed directly at Earth. Together, these factors made for a flash at least 10 times brighter than all the others spotted in the three decades since such bursts were discovered, leading some astronomers to dub it the “BOAT”—brightest of all time.

“I kept thinking, is this real? Because if it is, it’s an extremely rare, once-in-a-lifetime type of event,” Fong says. She and others are in the thick of collecting data that they hope will confirm that the rays actually came from a supernova, and help them isolate which stellar properties led to such an energetic explosion and how much of the collapsing material got spat out by the infant black hole. (Theoretical musings have already started appearing on the arXiv preprint server.)

While detecting supernovae is now fairly common, it’s rarer to catch one in conjunction with a gamma-ray burst—they’re usually too faint to show up because they are so far away, and only a fraction of supernovae actually generate these explosions. But since this burst was so intense, scientists expect to see the supernova very clearly. “It’s really reinvigorated the community,” Fong says. “Everyone who has a telescope, even if they don’t normally study gamma-ray bursts, is trying to point their detectors at this to get the most complete dataset that we can.”

Gamma rays from the blast were recorded for several hundred seconds. Next came a slew of lower energy photons, including x-rays, optical and infrared light, and radio waves. It’s this afterglow that astronomers at ground-based telescopes are hungry to capture, because observing how the influx of photons changes over time will help them characterize the types of stars producing such bursts, the mechanisms driving these explosions, and the resulting environments they produce. These insights could shed light on what influence gamma-ray bursts have on future generations of stars, and determine whether stellar deaths make life possible for us on Earth by producing the heavy elements that can heat a planet’s interior and help sustain its magnetic field.

Because the emission spans nearly all wavelengths of light, many different instruments can observe it, which has turned the gamma-ray burst postmortem into a global scientific event. Orbiting satellites like NASA’s NuSTAR are measuring its high energy x-rays, while sites like the Australia Telescope Compact Array are collecting the burst’s radio emission. “If we don’t get data one night, we can pretty much guarantee that someone will,” says Jillian Rastinejad, a Northwestern graduate student working with Fong. Together, they’re spearheading observations of visible light from the burst using the Gemini South telescope in Chile, data that will be supplemented by measurements from the Lowell Discovery Telescope in Arizona, South Korea’s Bohyunsun Optical Astronomy Observatory, and India’s Devasthal Fast Optical Telescope. Even the James Webb Space Telescope got in on the action, as scientists reported the afterglow observed in infrared last Friday.

These photons will linger for months—maybe years—though they’re already starting to dim. But scientists are also looking forward to what comes after the afterglow: a spike in optical and infrared light coming from the apparent supernova itself, which will confirm that the gamma-ray burst was indeed triggered by a cataclysmic stellar death. This signal typically appears 14 to 20 days after the burst.

With this data, astronomers could potentially link the existence of heavy elements, like silver and gold, to the environment surrounding the blast. Back in 2017, they discovered that short gamma-ray bursts—those lasting just a couple of seconds—generated the hot, dense, and neutron-rich conditions necessary to forge heftier members of the periodic table, like thorium, which radioactively decays in Earth’s core to generate heat. But Brian Metzger, an astrophysicist at Columbia University, has a hunch that the longer bursts may do this, too. “We haven’t had many opportunities to see supernovae from these types of very energetic gamma-ray bursts,” he says, but this event will allow his theory to be tested. “So I expect some surprises.”

At some telescopes, data-taking days—or rather, nights—are numbered. Soon, the burst will be at a celestial position that’s no longer visible from these vantage points after the sun goes down. Those astronomers will move on to characterizing first detections of the blast. But analyzing the data may prove challenging, particularly for those studying the initial gamma-rays. “Our instruments are very sensitive, and they’re meant to detect faint sources,” says Judith Racusin, a deputy project scientist of the Fermi Space Telescope. But this time, millions of photons were collected. “They all kind of get jumbled together,” she says. “So instead of detecting the energy of each individual gamma ray, we detect the sum of the energy of those gamma rays.” That’ll make it hard to extract information from the true number of photons observed and their energies—one downside to the blast being so extraordinarily bright.

Still, astronomers know they’re lucky to have witnessed such an event. “It’s exciting to have such a monster in the backyard,” Metzger says. They expect to confirm stellar collapse as the genesis of the blast as soon as this week—though it will take time for those results to be published. In the meantime, they’ll continue to capture the glowing remnants of the explosion, share new observations with each other, and theorize about the physics behind what many are calling the celestial event of the century. “This is an ongoing effort,” Fong says. “And we’re just watching the story unfold.”

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