Brightest Gamma-Ray Burst Illuminates Our Galaxy As Never Before

ESA space telescopes have observed the brightest gamma-ray burst ever seen. Data from this rare event could become instrumental in understanding the details of the colossal explosions that create gamma-ray bursts (GRBs).

X-rays from the blast have illuminated 20 dust clouds in our galaxy, allowing their distances and dust properties to be determined more accurately than ever before. But a mystery remains. The debris from the exploded star that produced the gamma-ray burst appears to have disappeared without trace.

GRB 221009A was first reported when NASA’s Neil Gehrels Swift Observatory detected X-rays on 9 October 2022. The source appeared to be located in our Milky Way, not far from the galactic centre. However, more data from Swift and NASA’s Fermi Gamma-Ray Space Telescope soon suggested it was much further away. Observations from the European Southern Observatory’s Very Large Telescope then pinpointed the burst to a much more distant galaxy that happened to be behind our own.

Being much further away, around two billion light-years instead of several tens of thousands, meant that the GRB had to be exceptionally bright.

“The difference between your typical gamma-ray burst and this one is about the same as the difference between the light bulb in your living room and the lit-up floodlights in a sports stadium,” says Andrew Levan, Radbound University, the Netherlands, who used the NASA/ESA/CSA James Webb Space Telescope and the NASA/ESA Hubble Space Telescope to observe the burst.

Statistically, a GRB as bright as GRB 221009A is only expected to happen once in many thousands of years, it may even be the brightest gamma-ray burst since human civilisation began. Astronomers therefore dubbed it BOAT – the brightest of all time.

“This has been a very eye-opening event. We have been very lucky to witness it,” says Alicia Rouco Escorial, an ESA Research Fellow who studies GRBs.

Calculations show that for the few seconds it lasted, the blast deposited around a gigawatt of power into Earth’s upper atmosphere. That is the equivalent of a terrestrial power station’s energy output. “So many gamma rays and X-rays were emitted that it excited the ionosphere of the Earth,” says Erik Kuulkers, ESA Project Scientist for Integral, one of the spacecraft that detected the GRB.

A number of other ESA spacecraft, XMM-Newton, Solar Orbiter, BepiColombo, Gaia, and SOHO, also detected the GRB or its effects on our galaxy. The event was so bright that even today the residual radiation, known as the afterglow, is still visible and will remain so for a long time yet. “We will see the afterglow of this event for years to come,” says Volodymyr Savchenko, University of Geneva, Switzerland, who is currently analysing the Integral data.

This large amount of data from entirely different instruments is now being brought together to understand how the original explosion took place, and how the radiation has interacted with other matter on its journey through space.

One area that has already yielded scientific results is from the way the X-rays have illuminated dust clouds in our galaxy. The radiation travelled through intergalactic space for around two billion years before entering our galaxy. It then encountered the first dust cloud around 60 000 years ago, and the last one about 1000 years ago.

Each time the X-rays encountered a dust cloud, it scattered some of the radiation, creating concentric rings that appeared to expand outwards. ESA’s XMM-Newton observed these rings for several days after the GRB. The closest clouds produced the largest rings simply because they appear bigger by perspective.

Andrea Tiengo, Scuola Universitaria Superiore IUSS Pavia, Italy, and a team of astronomers have analysed the data to derive the most accurate distance to each of these dust clouds. “The first cloud it hit appears to be on the very edge of our galaxy, far from where galactic dust clouds are usually observed,” Andrea says. The team then inferred the properties of the dust grains in the clouds because the X-rays are scattered according to the size, shape and composition of the dust.

Over the years, astronomers have proposed a number of different properties for the dust grains and so Andrea and colleagues were able to test them against the X-ray data. They found that one model reproduced the rings extremely well. In this model, the dust grains were composed mostly of graphite, a crystalline form of carbon. They also used their data to reconstruct the X-ray emission from the GRB itself because that particular signal was not observed directly by any instrument.

But a mystery remains about the object that exploded to create the GRB. Andrew Levan and colleagues used the Webb and Hubble space telescopes to look for the aftermath of the explosion – and found nothing. “That’s weird,” he says, “and it’s not totally obvious what it means.”

It could be that the star was so massive that following the initial explosion, it immediately formed a black hole that swallowed the material that would traditionally make the gaseous cloud known as a supernova remnant.

So, there is a lot of follow-up work to be done as astronomers continue to search for the remains of the star that exploded. One thing they will look for is traces of heavy elements such as gold, which are thought to be produced in such massive explosions.