NASA missions study what may be a 1-in-10,000-year gamma-ray burst

An extraordinary gamma-ray burst (GRB) dubbed BARCO, the brightest of all time, by astronomers was 70 times brighter than any seen so far.

First observed in October 2022 as an intense pulse of radiation sweeping the solar system, the outburst, named GRB 221009A, triggered detectors on numerous spacecraft and observatories around the world followed. After analyzing all this data, astronomers, including experts from the University of Birmingham, can now characterize just how bright it was and better understand its scientific impact.

Articles describing their results are published today (March 28, 2023) in a focus issue of The Astrophysical Journal Letters, which conclude that GRB 221009A was the brightest burst of X-ray and gamma-ray energies since human civilization began.

Astronomers from the University of Birmingham participated in the analysis of data on GRB 221009A from the James Webb Space Telescope (JWST), its first use for this type of study, and from the Ultraviolet/Optical Telescope aboard NASA’s Neil Gehrels Swift Observatory ( swift). With this type of GRB, astronomers hope to find a bright supernova a few weeks later, but so far it has proven elusive.

Dr. Samantha Oates, a postdoctoral researcher in the University of Birmingham’s School of Physics and Astronomy, was involved in the analysis of Swift’s UV/optical data.

“Based on our data, this GRB appeared normal in terms of optical brightness,” he explained. “However, because it occurred behind our galaxy, the large amount of dust along its line of sight would have diminished its brightness. This dust could explain why the supernova has not been found. Had it been observed elsewhere in the sky, the GRB would have appeared about 40 times brighter in the visual band, much brighter than any other GRB observed to date.”

Dr Benjamin Gompertz, also from the University of Birmingham, was part of the team that used JWST to search for evidence of heavy elements within the burst.

He said: “GRBs like 221009A are powered by very massive stars that run out of fuel and collapse into black holes under their own gravity. The extremely hot material left behind by this process could be an important birth site for heavy elements like gold. JWST observations can detect telltale signatures of the formation of new heavy elements, teaching us about the cosmic origins of some of the most massive elements found in nature.”

Observations of the burst span the entire spectrum, from radio waves to gamma rays, and include data from many missions by NASA and its partners, 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.

Measurements indicate that the signal from GRB 221009A had been traveling for approximately 1.9 billion years before reaching Earth, making it one of the closest known “long” GRBs, with an initial emission lasting more than two seconds. Astronomers believe these outbursts represent the “birth cry” of a black hole that formed when the core of a massive star collapsed under its own weight. As it rapidly ingests surrounding matter, the black hole shoots out jets in opposite directions containing particles accelerating to nearly the speed of light. These jets streak through the star, emitting X-rays and gamma rays as they stream into space.

As the jets continue to expand into the material surrounding the doomed star, they produce a multi-wavelength afterglow that gradually fades.

The jets themselves weren’t unusually powerful, but they were exceptionally narrow, much like the stream configuration of a garden hose, and one was pointed directly at the ground. The closer in front we see a jet, the brighter it appears. Although the glow was unexpectedly dim at radio energies, GRB 221009A is likely to remain detectable for years, providing a new opportunity to track the full life cycle of a powerful aircraft.

The outburst also allowed astronomers to probe distant dust clouds in our own galaxy. As the fast X-rays traveled toward us, some of them were reflected off the dust layers, creating extended “light echoes” from the initial explosion in the form of rings of X-rays expanding from the location of the explosion. Swift’s X-ray telescope discovered the presence of a series of echoes. Detailed monitoring by ESA’s (European Space Agency) XMM-Newton telescope, together with data from Swift, revealed that these extraordinary rings were produced by 21 different dust clouds.

GRB 221009A is only the seventh gamma-ray burst to show X-ray rings, and it triples the number seen previously by about 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 central plane of the galaxy, where the solar system resides.

Lastly, the outburst offers the opportunity to explore a big cosmic question. “We think of black holes as all-consuming, but do they also put energy back into the universe?” asked Michela Negro, an astrophysicist at the University of Maryland, Baltimore County, and NASA’s Goddard Space Flight Center in Greenbelt.

His team was able to probe the dust rings with NASA’s Imaging X-ray Polarimetry Explorer to get a glimpse of how the rapid emission was organized, which may give insight into how the jets form. Furthermore, a small degree of polarization observed in the afterglow phase confirms that we saw the jet almost directly head-on.

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


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