Astronomers observe 'brightest ever' gamma-ray burst

In October 2022, an pulse of intense radiation swept through the solar system that was considered so exceptional that astronomers dubbed it the BOAT — the brightest of all time.

The gamma-ray burst (GRB), named GRB 221009A, triggered detectors on numerous spacecraft, and observatories around the globe followed up. After combing through all of this data, astronomers have claimed that the explosion is the brightest burst of X-ray and gamma-ray energies since human civilisation began. Indeed, an analysis of some 7000 GRBs — mostly detected by NASA’s Fermi Gamma-ray Space Telescope and the Russian Konus instrument on NASA’s Wind spacecraft — established that events this bright occur only once every 10,000 years.

The burst was so bright that it effectively blinded most gamma-ray instruments in space, which means they could not directly record the real intensity of the emission. US scientists were able to reconstruct this information from the Fermi data. They then compared the results with those from the Russian team working on Konus data and Chinese teams analysing observations from the GECAM-C detector on their SATech-01 satellite and instruments on their Insight-HXMT observatory. Together, they prove the burst was 70 times brighter than any yet seen.

Observations of the burst span the spectrum, from radio waves to gamma rays, and include data from many NASA and partner missions, including NASA’s Neil Gehrels Swift Observatory and the James Webb Space Telescope (JWST), and others such as the NICER X-ray telescope on the International Space Station, NASA’s NuSTAR observatory, and even Voyager 1 in interstellar space. Results have been published in The Astrophysical Journal Letters.

The measurements indicate the signal from GRB 221009A had been travelling for about 1.9 billion years before it reached Earth, making it among the closest-known ‘long’ GRBs whose initial, or prompt, emission lasts more than two seconds. Astronomers think these bursts represent the ‘birth cry’ of a black hole that formed when the core of a massive star collapsed under its own weight.

As it quickly ingests the surrounding matter, the black hole blasts out jets in opposite directions containing particles accelerated to near the speed of light. These jets pierce through the star, emitting X-rays and gamma rays as they stream into space. As the jets continue to expand into material surrounding the doomed star, they produce a multi-wavelength afterglow that gradually fades away.

Gamma-ray bursts are the most luminous explosions in the cosmos. Astronomers think most occur when the core of a massive star runs out of nuclear fuel, collapses under its own weight and forms a black hole, as illustrated in this animation. The black hole then drives jets of particles that drill all the way through the collapsing star at nearly the speed of light. These jets pierce through the star, emitting X-rays and gamma rays (magenta) as they stream into space. They then plough into material surrounding the doomed star and produce a multiwavelength afterglow that gradually fades away. Video credit: NASA’s Goddard Space Flight Center.

“Being so close and so bright, this burst offered us an unprecedented opportunity to gather observations of the afterglow across the electromagnetic spectrum and to test how well our models reflect what’s really happening in GRB jets,” said Kate Alexander, an assistant professor at the University of Arizona. “25 years of afterglow models that have worked very well cannot completely explain this jet. In particular, we found a new radio component we don’t fully understand. This may indicate additional structure within the jet or suggest the need to revise our models of how GRB jets interact with their surroundings.”

The jets themselves were not unusually powerful, but they were exceptionally narrow — much like the jet setting of a garden hose — and one was pointed directly at Earth, Alexander said. The closer to head-on we view a jet, the brighter it appears. And although the afterglow was unexpectedly dim at radio energies, it’s likely that GRB 221009A will remain detectable for years, providing a novel opportunity to track the full life cycle of a powerful jet.

With this type of GRB, astronomers expect to find a brightening supernova a few weeks later, but so far this has proven elusive. The University of Birmingham’s Dr Samantha Oates, who was involved in analysing optical/UV data from Swift, offered a potential explanation for this.

“From our data, this GRB looked ordinary in terms of its optical brightness. However, because it occurred behind our galaxy, the vast amount of dust along its line of sight would have diminished its brightness,” Oates said.

“This dust might explain why the supernova hasn’t been found. If it had been observed at another point in the sky, the GRB would have appeared around 40 times brighter in the visual band — much brighter than any other GRB observed to date.”

“We cannot say conclusively that there is a supernova, which is surprising given the burst’s brightness,” added Andrew Levan, a professor of astrophysics at Radboud University. “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.”

Birmingham’s Dr Benjamin Gompertz was part of the team using the JWST to look for evidence of heavy elements within the burst. He said, “GRBs like 221009A are powered by very massive stars running out of fuel and collapsing to black holes under their own gravity. The extremely hot material left behind by this process might be an important birth site for heavy elements like gold. Observations with JWST can detect tell-tale signatures of new heavy elements forming, teaching us about the cosmic origins of some of the most massive elements found in nature.”

The burst also enabled astronomers to probe distant dust clouds in our own galaxy. As the prompt X-rays travelled toward us, some of them reflected off dust layers, creating extended ‘light echoes’ of the initial blast in the form of X-ray rings expanding from the burst’s location. The X-ray telescope on Swift discovered the presence of a series of echoes. Detailed follow-up by the European Space Agency’s (ESA) XMM-Newton telescope, together with Swift data, revealed that these rings were produced by 21 distinct dust clouds.

“How dust clouds scatter X-rays depends on their distances, the sizes of the dust grains and the X-ray energies,” said Sergio Campana, Research Director at Brera Observatory and the National Institute for Astrophysics in Merate, Italy. “We were able to use the rings to reconstruct part of the burst’s prompt X-ray emission and to determine where in our galaxy the dust clouds are located.”

GRB 221009A is only the seventh gamma-ray burst to display X-ray rings, and it triples the number previously seen around one. The echoes came from dust located between 700 and 61,000 light-years away. The most distant echoes — clear on the other side of our Milky Way galaxy — were also 4600 light-years above the galaxy’s central plane, where the solar system resides.

Michela Negro, an astrophysicist at the University of Maryland and NASA’s Goddard Space Flight Center, said the burst also offers an opportunity to explore a big cosmic question: “We think of black holes as all-consuming things, but do they also return power back to the universe?” Her team was able to probe the dust rings with NASA’s Imaging X-ray Polarimetry Explorer to glimpse how the prompt emission was organised, which can give insights into how the jets form. In addition, a small degree of polarisation observed in the afterglow phase confirms that we viewed the jet almost directly head-on.

Together with similar measurements now being studied by a team using data from ESA’s INTEGRAL observatory, scientists say it may be possible to prove that the BOAT’s jets were powered by tapping into the energy of a magnetic field amplified by the black hole’s spin. Predictions based on such models have already successfully explained other aspects of this burst.

Image caption: XMM-Newton images recorded 20 dust rings, 19 of which are shown here in arbitrary colours. The image merges observations made two and five days after GRB 221009A erupted. Dark stripes indicate gaps between the detectors. A detailed analysis shows that the widest ring visible here, comparable to the apparent size of a full moon, came from dust clouds located about 1300 light-years away. The innermost ring arose from dust at a distance of 61,000 light-years, on the other side of our galaxy. Image credit: ESA/XMM-Newton/M Rigoselli (INAF).