For half a century, astronomers have studied gamma-ray bursts—brief, blinding flashes of high-energy light that erupt across the universe. These bursts, known as GRBs, are the most powerful explosions we know of, outshining entire galaxies for mere seconds or minutes before fading into darkness. They are cosmic catastrophes, usually signaling the death of a massive star or the collision of neutron stars. Yet in July 2025, a team of astronomers witnessed something so strange, so utterly unprecedented, that it has shaken the foundations of our understanding.
On July 2, NASA’s Fermi Gamma-ray Space Telescope detected a gamma-ray burst, later labeled GRB 250702B. But unlike every GRB observed before, this one didn’t fade in seconds or minutes—it went on for nearly an entire day, repeating its flashes like a cosmic heartbeat.
“This event is unlike any other seen in 50 years of GRB observations,” said Dr. Antonio Martin-Carrillo of University College Dublin, co-lead author of the discovery published in The Astrophysical Journal Letters. “GRBs are catastrophic events, so they are expected to go off just once because the source that produced them does not survive the dramatic explosion. This event baffled us, not only because it showed repeated powerful activity, but also because it seemed to be periodic, which had never been seen before.”
The Nature of Gamma-Ray Bursts
To appreciate just how extraordinary this discovery is, we need to understand what a typical GRB is. These explosions occur when a massive star collapses at the end of its life, forming either a black hole or a highly magnetized neutron star called a magnetar. The collapse funnels energy into narrow jets of gamma rays, which, if pointed toward Earth, appear as a sudden and brilliant burst of radiation.
GRBs are so powerful that in just a few seconds, they can release more energy than our Sun will produce over its entire lifetime. Yet they are fleeting, vanishing before telescopes can fully grasp their structure. That is why the discovery of GRB 250702B, lasting around 24 hours, is so perplexing.
“This burst was 100 to 1000 times longer than most GRBs,” explained Prof. Andrew Levan of Radboud University in the Netherlands, co-lead author of the study. “It rewrites what we thought we knew about these explosions.”
A Cosmic Puzzle of Origins
The event’s unusual longevity is not its only mystery. It also displayed repeating flashes, something unheard of in GRBs. Normally, once a star dies in such a cataclysm, there is nothing left to repeat. For GRB 250702B, astronomers considered two main possibilities.
The first is that it may have been caused by a massive star, perhaps 40 times the mass of our Sun, undergoing a very unusual kind of death. In this case, some material could have continued fueling the central engine, driving repeated bursts of energy long after the initial collapse.
The second possibility is even more exotic: a tidal disruption event (TDE), in which a star strayed too close to a black hole and was torn apart by its immense gravity. As the star’s gas spiraled inward, it may have produced periodic gamma-ray flares. But this too presented problems. To match the observed data, astronomers would need an unusual type of star being destroyed by a rare and elusive intermediate-mass black hole—a class of black holes that scientists have long theorized but struggled to confirm.
“Either option would be a first, making this event extremely unique,” said Dr. Martin-Carrillo.
Pinpointing the Source
Initially, the signal’s location was uncertain. NASA’s Fermi telescope and China’s Einstein Probe both detected activity, but they could only narrow down the region of the sky. At first glance, it even seemed like the event might be happening within our own Milky Way galaxy.
But then the European Southern Observatory’s Very Large Telescope (VLT), perched high in Chile’s Atacama Desert, entered the picture. Using its HAWK-I camera, astronomers pinpointed the event’s location far beyond our galaxy. Follow-up observations from the NASA/ESA Hubble Space Telescope confirmed the finding: GRB 250702B had originated in a distant galaxy, billions of light-years away.
“What we found was considerably more exciting—the fact that this object is extragalactic means that it is considerably more powerful,” said Dr. Martin-Carrillo.
The size and brightness of the host galaxy suggest that it may lie several billion light-years from Earth, though more precise measurements are still underway.
Watching the Aftermath
The discovery did not end with the initial detection. To unravel the mystery of GRB 250702B, astronomers turned to an array of powerful instruments. The VLT’s X-shooter spectrograph has been capturing the fading light of the explosion, while the James Webb Space Telescope—the most advanced space observatory ever built—is probing the event’s infrared signature to learn more about the environment in which it occurred.
“We keep collecting more data,” Dr. Martin-Carrillo explained. “Determining the true distance of this event will be key to measuring its true energy and will help us improve our physical modeling.”
Each observation adds a piece to the puzzle. Was this the death throes of a massive star unlike any ever seen? Or was it the long-awaited evidence of an intermediate-mass black hole? The answer could reshape astrophysics.
Why This Matters
To the untrained eye, a distant burst of gamma rays might seem far removed from our daily lives. But discoveries like this are cornerstones of our quest to understand the universe. GRB 250702B challenges everything we thought we knew about how stars die and how black holes behave. It also provides a rare opportunity to explore physical conditions we could never reproduce on Earth.
Such findings push science forward by forcing us to question established theories and expand the boundaries of possibility. They remind us that the cosmos still holds surprises beyond our imagination.
“This is an extremely unusual and exciting object,” said Dr. Martin-Carrillo. “We are still not sure what produced this or if we can ever really find out, but with this research, we have made a huge step forward.”
The Endless Sky of Mysteries
GRB 250702B is not just another data point in astronomy—it is a message from the cosmos, a reminder that the universe is stranger, more dynamic, and more alive than we ever dared to believe. Its repeating flashes are like a distant lighthouse, signaling across billions of years and billions of light-years, daring us to interpret what they mean.
For centuries, humanity has looked upward, asking the same simple question: What’s out there? With every discovery—whether it is a planet, a galaxy, or an explosion unlike any other—the universe answers with something more mysterious, more humbling, and more wondrous.
The story of GRB 250702B is far from over. As astronomers continue to watch its fading glow and unravel its secrets, one thing is certain: the universe is still full of surprises, and we are only just beginning to understand the cosmic symphony in which we play a part.
More information: Andrew J. Levan et al, The Day-long, Repeating GRB 250702B: A Unique Extragalactic Transient, The Astrophysical Journal Letters (2025). DOI: 10.3847/2041-8213/adf8e1
