Long-Lived Gamma-Ray Burst Is Unlike Any Seen Before

Astronomers are excited about this all-day gamma ray burst

By Yvette Cendès edited by Lee Billings

You might think astronomers have seen it all, but the universe still occasionally throws up new mysteries to keep us guessing.

On July 2, 2025, observers around the world were alerted to a new gamma-ray burst (GRB), GRB 250702B, detected by NASA’s Fermi Gamma-Ray space telescope. GRBs are extremely energetic explosions that are among the most powerful astrophysical events in the universe, so bright they can be seen billions of light years away. They are most often caused either by the merger of a pair of neutron stars, or by a very massive star ending its life in a supernova called a collapse; In both of these “classic” cases, the resulting stellar cataclysm can spew out a giant, tightly concentrated jet of radiation and particles. When one of these jets is pointed directly at our solar system, we consider it a GRB.

They are not that unusual to spot: the universe is so big and GRBs are so bright that astronomers see one or two per day on average. Despite everything, we quickly realized something very what was unusual happened with GRB 250702B. In the half-century or so of GRB studies, no one has ever seen one of these epic events last more than a few hours at most. However, almost a full day passed before GRB 250702B disappeared.

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“The most surprising thing about this event is the duration of the emission. If it came from a GRB, it is the longest ever detected,” says Adelle Goodwin, an astronomer at Curtin University in Australia, who is leading the follow-up radio observations with the Very Large Array in New Mexico. Another oddity was that this GRB appeared to repeat itself, with astronomers seeing what appeared to be three separate pulses over the course of almost a day; a repeat would be odd, as GRBs are normally so catastrophic that they effectively destroy their astrophysical sources, preventing any recurrence. Faced with a conundrum, scientists around the world rushed to point every possible telescope at GRB 250702B to gather more clues.

Astronomers are not the type to shy away from challenges, but GRB 250702B presented us with several. First, its location in the sky was in the star- and dust-filled galactic plane of the Milky Way, making its true cosmic origins unclear; there was a good chance that it had occurred in a distant part of the universe, with its source obscured by dust, but in principle, whatever caused this explosion could have been inside our galaxy. Fortunately, the James Webb Space Telescope was designed to peer into all that dust, allowing a team of researchers to reveal the truth in a recent preprint. The still mysterious origins of GRB 250702B date back to a galaxy located more than five billion light years away. Additionally, the galaxy itself is unusual: it is extremely large and dusty compared to most other galaxies and is in fact the largest and brightest GRB-hosting galaxy ever seen. Astronomers can’t say for sure yet, but an unusual GRB from an unusual galaxy could be a clue in itself to what really caused the explosion.

Of course, that’s the real question behind GRB 250702B: what causes it? Theorists currently propose two main possibilities. This very strange GRB could have resulted from the particularly unusual collapse of a massive star that launched an anomalously energetic jet, or it could have come from a tidal disruption event (TDE) in which a white dwarf star was torn apart and devoured by an intermediate-mass black hole weighing on the order of 100,000 solar masses. “The duration of GRB emissions cannot be explained by classic collapse scenarios, which means that more exotic scenarios must be invoked,” says Goodwin.

I recently attended this year’s meeting of the High Energy Astrophysics Division (HEAD) of the American Astronomical Society, where I presented some of my own research on TDEs; suffice it to say that speculation about the origin of GRB 250702B dominated much of the informal sessions. “It’s too long to be a GRB but not bright enough to be a TDE at this distance,” one of my colleagues confidently argued. Others were more cautious, emphasizing that extraordinary claims require extraordinary evidence, and said they believed the threshold had not yet been met for GRB 250702B. One thing is certain: nothing excites a crowd of astronomers quite like an unusual signal that none of us has seen before. (Despite all this enthusiasm, many astronomers unfortunately declined to comment for this story, citing an upcoming NASA press release about GRB 250702B which is stuck in limbo due to the ongoing U.S. federal government shutdown.)

There are, of course, even crazier possibilities: it could be that this event represents a fantastic and hitherto completely unprecedented phenomenon. For example, a recent preprint claimed that GRB 250702B was actually born from a black hole that merged with a star. This event would be extremely rare but is expected to occur on certain occasions, such as if two stars orbit as a pair and a larger one explodes, leaving behind a black hole. Over time, gravitational interactions would cause the remaining star and its black hole companion to spiral, culminating in a spectacular merger that would set off unprecedented astrophysical fireworks.

Whatever its origin, GRB 250702B is exceptional and could be the first direct evidence of a completely new type of gigantic space explosion. Astronomers are still busy putting together a complete picture of where this spatial oddity fits into our strange and exciting universe.

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