On October 9, 2022, the detectors of several space telescopes registered a huge shower of high-energy gamma rays from the distant cosmos – it was the brightest gamma ray flash ever observed. Now an international research team has been able to use the James Webb Space Telescope to reveal the cause of the unusual event. 1.9 billion years ago in a distant galaxy, a large, rapidly rotating star exploded as a supernovalike the scientists in the journal Nature Astronomy to report.
Due to the enormous brightness of GRB 221009A – as it is officially called – astronomers initially suspected that it was an explosion within our Milky Way. But further measurements showed that the gamma radiation comes from a distant galaxy. The assumption quickly arose that the gamma-ray burst was triggered by a supernova. But the search for debris from such a star explosion was initially unsuccessful, even with the largest telescopes.
This is no wonder, explains Peter Blanchard from Northwestern University in the USA: “The gamma-ray burst was so bright that its afterglow hopelessly outshone all traces of a supernova in the weeks and months after the eruption. So we had to wait for this afterglow had weakened enough to have a chance.” Blanchard and his team therefore pointed the James Webb telescope at the distant galaxy again six months after the gamma-ray burst appeared. And this time the researchers found what they were looking for: They discovered radiation from calcium and oxygen atoms – for the astronomers a clear indication of the explosion of a large star. But contrary to expectations, it was a completely normal stellar explosion. “With such a bright gamma-ray burst, one would also expect an unusually energetic supernova,” says Blanchard. “But that’s not the case: it was no brighter than other stellar explosions.”
The beam must have been highly focused and aimed directly at the earth
The researchers conclude that the strength of a gamma-ray burst is not linked to the intensity of the stellar explosion. It is assumed that the gamma radiation arises in a beam of material that is tightly focused by magnetic fields and shoots out into space when it explodes. In GRB 221009A, this beam must have been particularly focused and aimed precisely at the Earth to explain its brightness. An extremely rare combination, says Blanchard: “We can only see such an event on Earth every 10,000 years. So it is very fortunate that we now have the technology to record such eruptions.”
The team’s observations revealed another surprise: no traces of chemical elements heavier than iron were found in the remains of the exploded star. When the universe was created, the Big Bang, only hydrogen and helium as well as a little lithium were created. Heavier elements could only be formed through nuclear fusion inside stars – but only up to the element iron.
Astrophysicists have so far seen the source of the heaviest stable elements in the periodic table as the collision of neutron stars on the one hand and the explosion of large stars, as in the case of GRB 221009A, on the other. The detection of the supernova also offered Blanchard and his colleagues a unique opportunity to test this hypothesis. “But we found no evidence of such heavy elements,” reports Blanchard. Extreme events like GRB 221009A appear to be excluded as a source for the heaviest elements in the cosmos. The question of the origin of these elements therefore remains open, say the scientists.