Gamma ray bursts rank among the most energetic explosions known, usually flashing and fading within seconds or minutes, but on 2 July 2025 astronomers detected GRB 250702B, a source that produced repeated bursts for over seven hours and now stands as the longest gamma ray burst yet observed. Space based observatories, including NASAs Fermi Gamma ray Space Telescope, first recorded the gamma rays Los Angeles CA (SPX) Dec 09, 2025
Gamma ray bursts rank among the most energetic explosions known, usually flashing and fading within seconds or minutes, but on 2 July 2025 astronomers detected GRB 250702B, a source that produced repeated bursts for over seven hours and now stands as the longest gamma ray burst yet observed. Space based observatories, including NASAs Fermi Gamma ray Space Telescope, first recorded the gamma rays, localized the source in X rays, and triggered coordinated follow up at other wavelengths to determine its nature and environment.
Infrared observations with ESOs Very Large Telescope confirmed that GRB 250702B originated in a galaxy outside the Milky Way. A team led by Jonathan Carney at the University of North Carolina at Chapel Hill then moved to track the afterglow, the fading optical and infrared emission that carries information about the explosion and its surroundings.
To probe this record breaking event the team used the NSF Victor M Blanco 4 meter Telescope at Cerro Tololo Inter American Observatory together with the twin 8.1 meter International Gemini Observatory telescopes in Hawai'i and Chile. These facilities began observing roughly 15 hours after the initial detection and continued for about 18 days, with results now reported in The Astrophysical Journal Letters.
The Blanco telescope hosted the NEWFIRM wide field infrared imager and the 570 megapixel DOE fabricated Dark Energy Camera, while Gemini North and Gemini South used their Gemini Multi Object Spectrographs. Carney noted that rapid scheduling of Blanco and Gemini on short notice was essential for capturing transient events such as this burst, because without that responsiveness astronomers would miss key phases of distant explosions.
The observations showed that GRB 250702B was not visible in standard optical bands because of heavy dust extinction in both the Milky Way and especially its host galaxy. Gemini North provided the only detection near visible wavelengths of the host galaxy, and even then required nearly two hours of exposure time to extract a faint signal through the dust along the line of sight.
Researchers combined these data with new measurements from the Keck I Telescope at W M Keck Observatory, along with publicly available observations from the Very Large Telescope, NASAs Hubble Space Telescope, and several X ray and radio facilities. They compared this multiwavelength data set with theoretical models of relativistic jets and afterglows, using the models to test how different explosion scenarios would reproduce the observed light.
Analysis indicates that the initial gamma rays likely came from a narrow, high speed relativistic jet plowing into surrounding material. The team inferred that the host galaxy is extremely massive compared with typical GRB hosts and that the burst lies within a dense, dust rich region, possibly a thick dust lane that lies directly between Earth and the source, providing strong constraints on the system that produced the event.
Only about half a dozen of the roughly 15000 gamma ray bursts recorded since their discovery in 1973 approach the duration of GRB 250702B, and their proposed origins include collapsing blue supergiant stars, tidal disruption events, and newborn magnetars. GRB 250702B does not fit neatly into these categories, so the team considered several new progenitor scenarios consistent with the data.
Possible origins include a black hole plunging into a star stripped of hydrogen and composed mainly of helium, a star or sub stellar object being torn apart in a close encounter with a compact object such as a stellar mass black hole or neutron star in a micro tidal disruption event, or a star being disrupted as it falls into an intermediate mass black hole with a mass between about one hundred and one hundred thousand times that of the Sun. If the last scenario proves correct, GRB 250702B would mark the first time astronomers have seen a relativistic jet from an intermediate mass black hole actively consuming a star.
Further observations will be required to distinguish between these possibilities, but current measurements remain consistent with several of these novel explanations. Carney described the work as a kind of cosmic archaeology in which scientists reconstruct an event that occurred billions of light years away, and emphasized that uncovering such extreme phenomena shows how much remains to be learned about the most energetic events in the Universe.
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