Astronomers have observed a gamma-ray burst (GRB) unlike any seen before: a powerful explosion that repeated multiple times over a single day. The source of the radiation was pinpointed outside the galaxy by the European Southern Observatory's Very Large Telescope (VLT). The study was published in The Astrophysical Journal Letters.
GRB 250702B, an unusually long and repeating gamma-ray burst. Image Credit: European Southern Observatory
While GRBs are the most powerful explosions in the universe, this newly observed event, still shrouded in mystery, doesn't align with any known astronomical models. This GRB is “unlike any other seen in 50-years of GRB observations,” says Antonio Martin-Carrillo, Astronomer at University College Dublin, Ireland, and co-lead author of the study.
While gamma-ray bursts (GRBs) are known as the universe's most energetic explosions, typically lasting mere milliseconds to minutes and caused by events like the collapse of massive stars, the newly detected GRB 250702B is an anomaly. This powerful signal remarkably persisted for about a full day, an event unlike any previously recorded.
This is 100-1000 times longer than most GRBs.
Andrew Levan, Astronomer and Study Co-Lead Author, Radboud University, The Netherlands
“More importantly, gamma-ray bursts never repeat since the event that produces them is catastrophic,” said Martin-Carrillo.
The initial alert for this event came from NASA’s Fermi Gamma-ray Space Telescope, which detected three bursts from the same source on July 2nd. Later analysis revealed that the source had already shown activity nearly a day earlier, detected by the Einstein Probe, an X-ray space telescope operated by the Chinese Academy of Sciences in collaboration with the European Space Agency (ESA) and the Max Planck Institute for Extraterrestrial Physics. A gamma-ray burst of such duration and repetition had never been observed before.
Initial observations provided only an approximate location for the gamma-ray burst within the crowded plane of the galaxy, so astronomers turned to the European Southern Observatory’s Very Large Telescope (VLT). The VLT’s high-resolution capabilities were essential for pinpointing the exact source of the explosion among the countless stars.
“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 Levan, who is also affiliated with the University of Warwick, UK.
Astronomers found evidence that the source of the explosion was located in another galaxy using the HAWK-I camera on the Very Large Telescope. This finding was later confirmed by 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.
Antonio Martin-Carrillo, Astronomer, University College Dublin
While the size and brightness of the host galaxy suggest it is located a few billion light-years away, more data is needed to confirm this distance.
The true nature of the event remains a mystery, although one possible scenario is the collapse of a massive star onto itself, which would release a tremendous amount of energy.
“If this is a massive star, it is a collapse unlike anything we have ever witnessed before,” said Levan, as in that case the GRB would have lasted just a few seconds.
An alternative theory suggests a star was ripped apart by a black hole, but this would require an unlikely combination of an unusual star and an even more unusual black hole to explain the event's full properties.
The research team is now monitoring the explosion's aftermath using advanced instruments like the VLT’s X-shooter spectrograph and the James Webb Space Telescope. The discovery that this GRB occurred in another galaxy will be crucial for finally deciphering its cause.
We are still not sure what produced this, but with this research we have made a huge step forward towards understanding this extremely unusual and exciting object.
Antonio Martin-Carrillo, Astronomer, University College Dublin
Zooming into an unusually long and repeating explosion
Zooming into an unusually long and repeating explosion. Video Credit: ESO/L. Calçada/N. Risinger (skysurvey.org)/Digitized Sky Survey 2/VISTA Hemisphere Survey/A. Levan, A. Martin-Carrillo et al. Music: Azul Cobalto
Journal Reference:
Levan, J. A., et al. (2025) The Day-long, Repeating GRB 250702B: A Unique Extragalactic Transient. The Astrophysical Journal Letters. doi.org/10.3847/2041-8213/adf8e1