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

Astronomers are alarmed by this daylong gamma-ray burst

TO Yvette Sendes edited by Lee Billings

You might think astronomers have seen it all, but the universe still throws new mysteries at us from time to time to keep us on our toes.

On July 2, 2025, observers around the world were alerted to a new gamma-ray burst (GRB). GRB 250702Bdiscovered by NASA's Fermi Gamma-ray Space Telescope. GRBs are extremely energetic explosions that are classified as most powerful astrophysical events in the Universeso bright that they can be seen billions of light years away. Most often they arise from either the merger of a pair of neutron stars or a very massive star ending its life in a supernova called a collapsar; in both of these “classical” cases, the resulting stellar cataclysm can unleash a gigantic, tightly focused jet of radiation and particles. When one of these jets is aimed directly at our solar system, we see it as GRB.

They don't What unusual to find – The universe is so large and gamma-ray bursts so bright that astronomers see an average of one or two gamma-ray bursts per day. Despite this, we soon realized something Very something unusual happened to GRB 250702B. In roughly half a century of gamma-ray burst research, no one has ever seen one of these epic events last 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 radiation. If it comes from a gamma-ray burst, it is the longest ever detected,” explains Adele Goodwin, an astronomer at Curtin University in Australia who is leading follow-up radio observations of the Very Large Massive System in New Mexico. Another oddity was that this gamma-ray burst seemed to repeat itself: astronomers saw three separate pulses over the course of almost a day; a recurrence would be strange because gamma-ray bursts are usually so catastrophic that they effectively destroy their astrophysical sources, preventing any recurrence. Faced with a mystery, scientists around the world tried to point every telescope at GRB 250702B to gather more clues.

Astronomers are not one to shy away from solving problems, but GRB 250702B presented us with several of them. First, its location in the sky was in the star- and dust-filled galactic plane of the Milky Way, making its true cosmic origin unclear; Most likely, it occurred in some distant part of the Universe, and its source was hidden by dust, but in principle, what caused the explosion could be located inside our galaxy. Luckily, the James Webb Space Telescope was designed to peer through all that dust, allowing the team to reveal the truth in a recent preprint. The still mysterious origin of GRB 250702B dates back to a galaxy more than five billion light-years away. Moreover, the galaxy itself is unusual: it is extremely large and dusty compared to most other galaxies, and is in fact the largest and brightest galaxy with a GRB that anyone has ever seen. Astronomers can't say for sure yet, but an unusual gamma-ray burst coming from an unusual galaxy could itself be a clue to what actually caused the explosion.

Of course, the real question behind GRB 250702B is: what caused it? Currently, theorists offer two main possibilities. This very strange gamma-ray burst could be the result of a particularly unusual collapse of a massive star that released an anomalously energetic jet, or it could instead be due to tidal disruption event (TDE), in which a white dwarf star was torn apart and consumed by an intermediate-mass black hole of about 100,000 solar masses. “The duration of GRB emissions cannot be explained by classical collapsar scenarios, meaning more exotic scenarios must be used,” explains Goodwin.

I recently attended this year's meeting of the High Energy Astrophysics Division (HEAD) of the American Astronomical Society, presenting some of my own research on TDE; 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 stated. Others were more cautious, pointing out that extraordinary claims require extraordinary evidence, and said they believed the GRB 250702B threshold had not yet been reached. One thing is for sure: nothing excites a crowd of astronomers more than an unusual signal that none of us have seen before. (Despite all this excitement, many astronomers unfortunately declined to comment on the record for this story, citing NASA's upcoming press release about GRB 250702B, which is stuck in limbo due to the ongoing US federal government shutdown.)

There are, of course, wilder possibilities – perhaps this event represents some kind of fantastic and hitherto completely unknown phenomenon. For example, a recent preprint claimed that GRB 250702B actually originated from black hole merging with a star. This event would be extremely rare, but should happen in some cases, such as if two stars orbit in pairs and the larger star explodes, leaving behind a black hole. Over time, gravitational interactions will cause the remaining star and its black hole companion to spin together, resulting in a spectacular merger that will set off unprecedented astrophysical fireworks.

Whatever its origin, GRB 250702B is unique and may be the first direct evidence of an entirely new type of giant cosmic explosion. Astronomers are still busy unraveling the full picture of where this cosmic oddity fits into our strange and fascinating Universe.

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