Header image: The orange dot at the centre is the powerful explosion that repeated several times over the course of a day. Credit: ESO/A. Levan, A. Martin-Carrillo et al.
Astronomers have detected an explosion of gamma rays that repeated several times over the course of a day, an event unlike anything ever witnessed before. The source of the powerful radiation was discovered to be outside our galaxy, its location pinpointed by the European Southern Observatory’s Very Large Telescope (VLT). Gamma-ray bursts (GRBs) are the most powerful explosions in the universe, normally caused by the catastrophic destruction of stars. But no known scenario can completely explain this new GRB, whose true nature remains a mystery.
GRBs are produced in catastrophic events like dying stars exploding in powerful blasts or stars being ripped apart by black holes. These celestial flashes of gamma-rays usually last just milliseconds to minutes, but this signal—GRB 250702B—lasted about a day.
“This immediately alerted us to the unusual nature of this explosion,” said Tanmoy Laskar, assistant professor in the Department of Physics & Astronomy at the University of Utah and co-author of a study on this event recently published in The Astrophysical Journal Letters.
The initial alert about this GRB came on July 2, 2025, from NASA’s Fermi Gamma-ray Space Telescope. Fermi detected not one, but three bursts from this source over the course of several hours. Retrospectively, it was also discovered that the source had been active almost a day earlier, as seen by the Einstein Probe, an X-ray space telescope mission by the Chinese Academy of Sciences with the European Space Agency (ESA) and the Max Planck Institute for Extraterrestrial Physics. Such a long and repeating GRB has never been seen before.
The gamma-ray discovery only gave an approximate location—in a very crowded part of the sky filled with stars from our Milky Way—making it difficult to locate the source of the flash. To pinpoint the precise position of its origin, the team turned to ESO’s VLT.
“Before these observations, the general feeling in the community was that this GRB must have originated from within our galaxy. The VLT fundamentally changed that paradigm,” said Andrew Levan, astronomer at Radboud University, The Netherlands, and co-lead author of the study.
Using the VLT’s HAWK-I camera, they found evidence that the source may actually reside in another galaxy and later confirmed this using the NASA/ESA Hubble Space Telescope.
“What we found was considerably more exciting: the fact that this object is extragalactic means that it is considerably more powerful,” said Antonio Martin-Carrillo, astronomer at University College Dublin, Ireland, and co-lead author of the study. The size and brightness of the host galaxy suggest it may be located a few billion light-years away, but more data are needed to refine this distance.
The nature of the event that caused this GRB is still unknown. One possible scenario is a massive star collapsing onto itself, releasing vast amounts of energy in the process.
“Just like other GRBs, this event also left behind lower-energy light cascading across the spectrum, all the way from X-rays to radio waves,” said Laskar. “Traditional collapsing-star models seem to be able to explain this residual, fading light, but the still-unknown distance to the event makes it difficult to be sure.”
Alternatively, a star being ripped apart by a black hole could produce a day-long GRB, but to explain other properties of the explosion would require an unusual star being destroyed by an even more unusual black hole.
To learn more about this GRB, the team has been monitoring the aftermath of the explosion with different telescopes and instruments, including the VLT’s X-shooter spectrograph and the James Webb Space Telescope, a joint project of NASA, ESA and the Canadian Space Agency. Measuring the distance to the galaxy that hosted the event will be key to deciphering the cause behind the explosion.
Adapted from the original post by the European Southern Observatory.
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This research was presented in the paper “The day long, repeating GRB 250702B: A unique extragalactic transient” (doi: 10.3847/2041-8213/adf8e1), published in The Astrophysical Journal Letters on August 29, 2025.
Find a full list of coauthors here.
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