An extraordinary gamma-ray burst (GRB) dubbed the BOAT – brightest of all time – by astronomers was 70 times brighter than any yet seen.
First observed in October 2022, as an intense pulse of radiation sweeping through the solar system, the burst, named GRB 221009A, triggered detectors on numerous spacecraft, and observatories around the globe followed up. After combing through all of this data, astronomers, including experts at the University of Birmingham, can now characterize just how bright it was and better understand its scientific impact.
Papers describing their results are published today [March 28 2023] in a focus issue of The Astrophysical Journal Letters, which conclude GRB 221009A was the brightest burst of X-ray and gamma-ray energies to occur since human civilisation began.
Astronomers from the University of Birmingham were involved in analysing data on GRB 221009A from the James Webb Space Telescope (JWST), its first use for this kind of study, and from the Ultra-violet/Optical Telescope onboard NASA’s Neil Gehrels Swift Observatory (Swift). With this type of GRB, astronomers expect to find a brightening supernova a few weeks later, but so far it has proven elusive.
Dr Samantha Oates, a postdoctoral research fellow in the University of Birmingham’s School of Physics and Astronomy, was involved in analysing optical/UV data from Swift.
“From our data, this GRB looked ordinary in terms of its optical brightness,” she explained. “However, because it occurred behind our galaxy, the vast amount of dust along its line of sight would have diminished its brightness. 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.”
Dr Benjamin Gompertz, also of the University of Birmingham, was part of the team using 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.”
Observations of the burst span the spectrum, from radio waves to gamma rays, and include data from many NASA and partner missions, including Swift and 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.
The measurements indicate the signal from GRB 221009A had been traveling 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 multiwavelength afterglow that gradually fades away.
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. The closer to head-on we view a jet, the brighter it appears. 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.
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 of 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 ESA’s (the European Space Agency’s) XMM-Newton telescope, together with Swift data, revealed these extraordinary rings were produced by 21 distinct dust clouds.
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 4,600 light-years above the galaxy’s central plane, where the solar system resides.
Lastly, the burst 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?” asked Michela Negro, an astrophysicist at the University of Maryland, Baltimore County, and NASA’s Goddard Space Flight Center in Greenbelt.
Her team was able to probe the dust rings with NASA’s Imaging X-ray Polarimetry Explorer to glimpse how the prompt emission was organized, which can give insights into how the jets form. In addition, a small degree of polarization 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.