Spatially resolved properties of the host galaxy GRB 250702B and supernova boundaries. Credit: arXiv (2025). DOI: 10.48550/arxiv.2509.22778
Given the immense size of the universe, it is not surprising that space still holds many secrets for us. Recently, astronomers believe they have discovered a kind of never-before-seen cosmic explosion, challenging what we thought we knew about how stars die.
In an article available on the arXiv preprint server, scientists report 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 typically last from 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, such as black holes or neutron stars, merge. GRB events are incredibly bright and can briefly eclipse entire galaxies.
However, GRB 250702B continued to ignite for an entire day. Normal cosmic explosions only happen once: you cannot explode a star twice. So what was going on? To find out, astronomers needed to know its distance, which could help them calculate its power and refine the list of possible causes.
The team used the powerful James Webb Space Telescope to calculate the distance, and by incorporating this measurement into the energy calculations, they discovered that it was the most energetic cosmic explosion ever recorded. Since most long gamma-ray bursts are accompanied by a massive supernova, astronomers looked for one nearby. However, they were unable to find a bright supernova, although a fainter supernova might be hidden by dust from the host galaxy.
“Our observations confirmed that GRB 250702B is a surprisingly distant event given the observed luminosity of its host galaxy. The associated energy release is sufficient to strain, but not definitively, canonical models of GRB collapse,” the researchers write.
Possible causes
The study authors speculate that GRB 250702B was caused by either a very unusual form of star collapse or a black hole destroying a small star. Another surprising discovery is that the host galaxy is extremely large and incredibly dusty. Usually, GRBs occur in small, young star-forming galaxies. This could mean that the environment played an important role 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 played an important role in the progenitor channel creating GRB 250702B,” the authors conclude.
So there are still many mysteries to be solved regarding GRB 250702B. This includes searching for the hidden supernova, long-term monitoring of the GRB afterglow, and creating new models to explain this rare event.
Written for you by our author Paul Arnold, edited by Gaby Clark, and fact-checked and revised by Robert Egan, this article is the result of painstaking human work. We rely on readers like you to keep independent science journalism alive. If this reporting interests you, consider making a donation (especially monthly). You will get a without advertising account as a thank you.
More information:
Benjamin P. Gompertz et al, JWST spectroscopy of GRB 250702B: an extremely rare and exceptionally energetic explosion in a dusty and massive galaxy at z = 1.036, arXiv (2025). DOI: 10.48550/arxiv.2509.22778
Journal information:
arXiv
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Quote: Webb follow-up observations confirm GRB 250702B is the most energetic cosmic explosion ever recorded (2025, October 17) retrieved October 18, 2025 from https://phys.org/news/2025-10-webb-grb-250702b-energetic-cosmic.html
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