Follow-up observations by Webb confirm GRB 250702B is most energetic cosmic explosion ever recorded
Paul Arnold
contributing writer
Gaby Clark
scientific editor
Robert Egan
associate editor
Considering the immense size of the universe, it's no surprise that space still holds plenty of secrets for us. Recently, astronomers believe they stumbled upon a kind of cosmic blast never seen before, and it's challenging what we thought we knew about how stars die.
In a paper on the arXiv preprint server, scientists report the longest gamma-ray burst (GRB) ever recorded. This unique event was spotted by NASA's Fermi telescope on July 2, 2025, and is called GRB 250702B.
Gamma-ray bursts are the most powerful and violent explosions in the universe and usually last a few milliseconds to several minutes. They occur when the core of a massive star collapses into a black hole or when two compact objects, like black holes or neutron stars, merge. GRB events are incredibly bright and can briefly outshine entire galaxies.
However, GRB 250702B kept flaring up for a full day. Normal cosmic explosions only happen once—you can't explode a star twice. So what was going on? To find out, the astronomers needed to know its distance, which could help them calculate its power and narrow down the list of possible causes.
The team used the powerful James Webb Space Telescope to work out the distance, and by incorporating this measurement into energy calculations, they found it was the most energetic cosmic explosion ever recorded. Since most long gamma-ray bursts are accompanied by a massive supernova, the astronomers looked for one nearby. However, they couldn't find a bright supernova, although a fainter one may be hidden by the host galaxy's dust.
"Our observations have confirmed GRB 250702B to be a surprisingly distant event given the observed brightness of its host galaxy. The associated energy release is enough to strain, but not definitely break, canonical GRB collapsar models," the researchers write.
Possible causes
The study authors speculate that GRB 250702B was caused by either a very unusual form of a collapsing star or a black hole destroying a small star. Another surprising finding was that the host galaxy is extremely large and incredibly dusty. Usually, GRBs occur in small, young star-forming galaxies. This could mean the environment was important in creating the unusual GRB, as the astronomers point out in their paper.
"The identification of such an exotic GRB in such an unusual galaxy raises the possibility that the environment was important in the progenitor channel creating GRB 250702B," the authors conclude.
So there are still plenty of mysteries to solve about GRB 250702B. This includes hunting for the hidden supernova, long-term monitoring of the GRB's afterglow and creating new models to explain this rare event.
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More information: Benjamin P. Gompertz et al, JWST Spectroscopy of GRB 250702B: An Extremely Rare and Exceptionally Energetic Burst in a Dusty, Massive Galaxy at z=1.036, arXiv (2025).
Journal information: arXiv
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