Scientists have detected an incredibly powerful burst of gamma radiation that repeated several times over a single day … and they can’t quite explain it. EarthSky’s Will Triggs has the story in this short video.
- Astronomers have spotted a mysterious gamma-ray burst. It’s unlike any we’ve detected before.
- Most gamma-ray bursts last just minutes at the most. They occur when a massive star dies or is ripped apart.
- But this gamma-ray burst lasted for a day. That’s unprecedented. So what was the source of this mysterious gamma-ray burst?
ESO published this original story on September 9, 2025. Edits by EarthSky.
Mysterious gamma-ray burst is unlike any detected before
Astronomers have detected an explosion of gamma rays that repeated several times over the course of a day, an event unlike anything we’ve witnessed before. The source of the powerful radiation was outside our galaxy, its location pinpointed by the European Southern Observatory’s Very Large Telescope (VLT). Gamma-ray bursts 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 gamma-ray burst, whose true nature remains a mystery.
Antonio Martin-Carrillo, an astronomer at University College Dublin in Ireland is the co-lead author of a study on this signal, published in The Astrophysical Journal Letters on August 29, 2025. Martin-Carrillo said this gamma-ray burst is:
unlike any other seen in 50 years of gamma-ray burst observations.
A one-of-a-kind gamma-ray burst
Gamma-ray bursts are the most energetic explosions in the universe. Catastrophic events – like massive stars dying in powerful blasts or black holes ripping apart stars – can produce gamma-ray bursts. They usually last milliseconds to minutes. But this signal – GRB 250702B – lasted about a day. Andrew Levan, astronomer at Radboud University, The Netherlands, and co-lead author of the study, said:
This is 100 to 1,000 times longer than most gamma-ray bursts.
Martin-Carrillo added:
More importantly, gamma-ray bursts never repeat since the event that produces them is catastrophic.
The initial alert about this gamma-ray burst 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, astronomers also discovered the source had been active almost a day earlier. 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 – made these earlier observations. It’s the first time such a long and repeating gamma-ray burst has been seen.
These observations only provided an approximate location for the gamma-ray burst. And that location was toward the plane of our galaxy, crowded with stars. Therefore, the team turned to ESO’s VLT to pinpoint the actual source within this area. Levan said:
Before these observations, the general feeling in the community was that this gamma-ray burst must have originated from within our galaxy. The VLT fundamentally changed that paradigm.
Pinpointing the mysterious gamma-ray burst
Using the VLT’s HAWK-I camera, they found evidence that the source may actually reside in another galaxy. This was later confirmed by the NASA/ESA Hubble Space Telescope. Martin-Carrillo said:
What we found was considerably more exciting: the fact that this object is extragalactic means that it is considerably more powerful.
The size and brightness of the host galaxy suggest it may lie a few billion light-years away. But scientists will need more data to refine this distance.
This video zooms in on the mysterious gamma-ray burst GRB 250702B. It took place in another galaxy, but its cause remains unknown. Video via ESO/ L. Calçada/ N. Risinger (skysurvey.org)/ Digitized Sky Survey 2/ VISTA Hemisphere Survey/ A. Levan, A. Martin-Carrillo et al. Music: Azul Cobalto.
Why did it occur?
The nature of the event that caused this gamma-ray burst is still unknown. One possible scenario is a massive star collapsing onto itself, releasing vast amounts of energy in the process. Levan said:
If this is a massive star, it is a collapse unlike anything we have ever witnessed before.
That’s because the gamma-ray burst from a massive star collapse would have lasted just a few seconds. Alternatively, a star being ripped apart by a black hole could produce a day-long gamma-ray burst. 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 gamma-ray burst, 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. Finding that this explosion took place in another galaxy will be key to deciphering what caused it. Martin-Carrillo said:
We are still not sure what produced this, but with this research we have made a huge step forward toward understanding this extremely unusual and exciting object.
Bottom line: Astronomers have spotted a mysterious gamma-ray burst that lasted for a day. So far, all other gamma-ray bursts astronomers have seen have lasted a few minutes at most. What caused this unique event?
Source: The Day-long, Repeating GRB 250702B: A Unique Extragalactic Transient
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