You might think astronomers have seen it all, but the universe still throws us occasional new mysteries to keep us on our toes.<\/p>\n
On July 2, 2025, observers around the world were alerted to a new gamma-ray burst (GRB), GRB 250702B<\/a>, detected by NASA\u2019s Fermi Gamma-Ray Space Telescope. GRBs are extremely energetic explosions that rank among the most powerful astrophysical events in the universe<\/a>, so luminous they can be seen from billions of light-years away. They\u2019re most commonly caused by either a merging pair of neutron stars or a very massive star ending its life in a supernova called a collapsar; in both of these \u201cclassical\u201d cases, the resulting stellar cataclysm can spit out a giant, tightly focused jet of radiation and particles. When one of these jets is pointed directly at our solar system, we see it as a GRB<\/a>.<\/p>\n They aren\u2019t that unusual to spot\u2014the universe is so big and GRBs are so bright that astronomers see one or two a day on average. Even so, we soon realized something very unusual was happening with GRB 250702B. In the roughly half century of GRB studies, no one has ever seen one of these epic events last more than a few hours at most. Yet almost a full day elapsed before GRB 250702B faded away.<\/p>\n [Sign up for Today in Science, a free daily newsletter]<\/a><\/p>\n \u201cThe most surprising thing about this event is the duration of the emission. If it came from a GRB, it\u2019s the longest one ever detected,\u201d explains Adelle Goodwin, an astronomer at Curtin University in Australia, who is leading follow-up radio observations with the Very Large Array in New Mexico. Another oddity was that this GRB seemed to repeat, with astronomers seeing what appeared to be three distinct pulses across its nearly day-long duration; repetition would be strange because GRBs are normally so catastrophic that they effectively destroy their astrophysical sources, preventing any recurrence. Confronted with an enigma, scientists around the world scrambled to point every telescope they could at GRB 250702B to gather more clues.<\/p>\n Astronomers are not ones to shy away from a challenge, but GRB 250702B has provided us with several. First, its on-sky location fell within the star-and-dust-packed galactic plane of the Milky Way, making its true cosmic origins unclear; odds were that it occurred in some distant part of the universe, with its source obscured by dust, but in principle, whatever created the burst could have been inside our galaxy. Luckily, the James Webb Space Telescope was designed to peer through all that dust, allowing a team of researchers to reveal the truth in a recent preprint<\/a>. GRB 250702B\u2019s still-mysterious origins trace back to a galaxy more than five billion light-years away from us. What\u2019s more, the galaxy itself is unusual: it\u2019s extremely large and dusty compared to most other galaxies and in fact is the biggest and brightest GRB-hosting galaxy anyone has ever seen. Astronomers can\u2019t yet say with certainty, but an unusual GRB coming from an unusual galaxy might be a clue in itself on what actually caused the explosion.<\/p>\n Of course, this is the real question behind GRB 250702B: What caused it? Theorists presently offer two main possibilities. This very odd GRB could have arisen from a particularly unusual collapse of a massive star that launched an abnormally energetic jet, or instead it could have come from a tidal disruption event<\/a> (TDE) in which a white dwarf star was ripped apart and devoured by an intermediate-mass black hole weighing on the order of 100,000 solar masses. \u201cThe duration of GRB emission cannot be explained by classical collapsar scenarios, which means more exotic scenarios need to be invoked,\u201d Goodwin explains.<\/p>\n I recently attended this year\u2019s meeting of the American Astronomical Society\u2019s High-Energy Astrophysics Division (HEAD), presenting some of my own research on TDEs; suffice to say that speculation on GRB 250702B\u2019s origin dominated much of the informal sessions. \u201cIt\u2019s too long to be a GRB but not luminous enough to be a TDE at that distance,\u201d one of my colleagues confidently argued. Others were more cautious, pointing out that extraordinary claims require extraordinary evidence, and said they felt that threshold was not yet met for GRB 250702B. One thing is for sure: nothing quite drives excitement in a crowd of astronomers like an unusual signal none of us has seen before. (Despite all that excitement, many astronomers unfortunately declined to comment on the record for this story, citing a forthcoming NASA press release on GRB 250702B that is stuck in limbo because of the ongoing U.S. federal government shutdown.)<\/p>\n