1.9 billion years ago, in a galaxy far, far away, there was a special event. The high-energy gamma rays released in the process hit the earth a little later. More precisely on October 9, 2022. It was the strongest gamma-ray burst ever recorded. Almost simultaneously, the two space telescopes Fermi and Swift automatically triggered the alarm.
Astronomers around the world began pointing their telescopes at the origin of the cosmic cataclysm. Several teams are now presenting the results of their observations in the specialist journal “Astrophysical Journal Letters”: The brightest gamma-ray burst of all time remains a mystery, there was no trace of an exploded star.
This gamma-ray burst was strange from the start, recalls Maia Willems from the Swift team: “It was much too bright, so I initially thought it had to be something else.” But since Fermi and other observatories had also registered the radiation burst, it was It quickly became clear: GRB 221009A – as it is officially called – was the strongest gamma-ray burst ever recorded.
Due to the enormous brightness, the researchers initially suspected that it was an explosion within our Milky Way, perhaps only a few tens of thousands of light-years away.
But measurements from the various astronomical instruments quickly showed that the source of the gamma-ray burst was much further away, in a galaxy some 1.9 billion light-years away. And so it was clear: The triggering explosion had to be powerful so that the flash could light up so brightly from this distance in the earthly sky.
In addition, the gamma radiation must have been tightly bundled towards Earth. This makes GRB 221009A an extremely rare event – Alicia Rouco Escorial from the European space agency Esa estimates that such a strong gamma-ray burst could only hit Earth every few thousand years: “So we were extremely lucky to experience such an event.”
Astrophysicists suspect two different causes of gamma-ray bursts. Brief flashes lasting less than two seconds are triggered by the collapse of an old, very massive star. This creates a black hole. On the other hand, longer flashes occur when two neutron stars or black holes merge, which also triggers gravitational waves.
The gamma-ray burst of October 9, 2022 belonged to the first category – it was brief and no gravitational waves were detected.
In both cases, part of the explosion energy shoots out into space in the form of gamma rays, among other things. Only when such a beam of radiation hits the earth do astronomers register a gamma-ray burst and thus receive information about the distant cosmic catastrophe. Calculations show that GRB 221009A transported a total of one gigawatt of energy into the upper atmosphere – briefly lighting up the ionosphere.
Similarly, the shower of gamma rays also lit up twenty dust clouds within the Milky Way – an opportunity for astrophysicists to study their nature. As observations with the European XMM-Newton space telescope show, the dust in these clouds consists mainly of graphite.
Now, half a year after the event, the astronomers are still regularly observing the region of origin of the gamma-ray burst. Because the afterglow of the explosion is still visible, and probably for a long time due to the strength of the flash.
“We will see the afterglow for years to come,” explains Volodymyr Savchenko of the University of Geneva. By observing the afterglow, the researchers hope to be able to track down the cause of the explosion.
That remains a mystery for the time being. Andrew Levan from Radbound University in the Netherlands and his colleagues report on their search for the remains of an exploded star with the Hubble and Webb Space Telescopes. They didn’t find anything, “and that’s crazy,” says Levan, “we just don’t know what it means.” One possible explanation would be that a black hole formed so quickly that any remnants of the blast were instantly engulfed.
The researchers now want to look specifically for heavy elements such as gold in the area, which can only occur in such explosions. GRB 221009 will keep scientists busy for a while.