The gamma and X-ray emissions from the burst provide new insight into how jets of material accelerate and can even illuminate 20 dust clouds in our galaxy. However, one mystery remains: the remains of the exploded star that caused the gamma-ray burst seem to have disappeared without a trace.
GRB 221009A, as the gamma-ray burst is called, was first reported when the Neil Gehrels Swift Observatory detected X-ray activity on 9 October 2022. The source was found to be in our Milky Way, not far from the galactic centre. However, more data from Swift and the Fermi Gamma-Ray Space Telescope soon suggested that it was much further away. Observations with the X-shooter instrument on ESO’s Very Large Telescope in Chile, led by Radboud University researcher Daniele Bjørn Malesani, later attributed the burst to a far more distant galaxy that happened to be behind ours. “The burst took place two billion light years away, but for a gamma-ray burst, that’s in our backyard. Such a powerful eruption, so close, that is a rare thing,” says Malesani.
The fact that the gamma-ray burst came from two billion light years away means it must have been exceptionally bright. “The difference between a typical burst and this one is about the same as the difference between the light bulb in your living room and the floodlights in a football stadium,” says Radboud University researcher Andrew Levan, who used the NASA/ESA James Webb Space Telescope to look for the aftermath of the explosion.
Statistically, a gamma-ray burst as bright as GRB 221009A occurs only once in many thousands of years. It may be the brightest burst of gamma rays since the dawn of human civilisation. Calculations show that for a few seconds, the gamma-ray burst delivered approximately one gigawatt of energy into the Earth’s upper atmosphere. That is the equivalent of the energy production of a terrestrial power plant. “So much gamma and X-ray radiation was emitted that it excited the Earth’s ionosphere,” said Erik Kuulkers, ESA project scientist for Integral, one of the telescopes that detected the gamma-ray burst.
A large amount of data from completely different instruments is now being brought together to understand how the original explosion occurred, and how the radiation interacted with other matter on its journey through space. One area that has already yielded results is the way in which the X-rays illuminated dust clouds in the Milky Way. The X-rays travelled through intergalactic space for about two billion years before entering the Milky Way. About 60,000 years ago, the X-rays encountered the first dust cloud, and about 1,000 years ago the last.
Whenever the X-rays encountered a dust cloud, the cloud scattered some of the radiation, creating concentric rings that seemed to extend outwards. ESA’s XMM-Newton observed these rings a few days after the gamma-ray burst. The nearest clouds yielded the largest rings, simply because the perspective makes them appear larger.
An Italian team analysed dust clouds and compared them with existing models. They found that one model reproduced the rings particularly well. In this model, the dust grains consisted mainly of graphite, a crystalline form of carbon. The researchers also used their data to reconstruct the X-ray emission from the gamma-ray burst itself, since this particular signal was not directly observed by any instrument.
But it remains a mystery which object exploded and caused the gamma-ray burst in the first place. Levan and his colleagues used the Webb telescope to search for the aftermath of the explosion – and found nothing. “That is strange,” he says, “and we are not entirely clear on what it means. It could be that the star was so heavy that a black hole was created immediately after the initial explosion. That black hole could have swallowed the material that would normally form the gas cloud known as a supernova remnant.”
There remains a lot of follow-up work to do. Astronomers will continue to search for the remains of the exploded star. Among other things, they will hunt for traces of heavy elements such as gold, which they believe are created in such heavy explosions.
