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On July 2, 2025, NASA's Fermi Gamma-ray Burst Monitor (Fermi-GBM) captured around three hours' worth of signals that appeared to come from the same source. When scientists compiled this data with signals picked up by multiple other instruments, like the Einstein Probe (EP) Wide-field X-ray Telescope and the Russian gamma-ray spectrometer, Konus-Wind, they found that they were dealing with the longest gamma ray burst (GRB) ever recorded. At around 25,000 seconds in duration (around seven hours), the GRB event scientists refer to as "GRB 250702B" beat out the prior record-holder, GRB 111209A, by 10,000 seconds.

Most GRBs detected in the past only lasted from less than a second up to a few minutes. So, such long-winded bursts of powerful gamma radiation in space are rare. However, these ultra-long gamma-ray bursts do happen, and for the most part, scientists have found reasonable explanations for them. Most long-lived GRBs have been linked to the collapse of massive stars, called collapsars, while short GRBs are tied to neutron star mergers. But, when scientists calculated the various properties of GRB 250702B, it didn't quite fit the mold of any previous GRB progenitor explanations.

In a new arXiv , a group of over 50 scientists joined to find out how, why, and where GRB 250702B came about. In the paper, the team analyzed all available data, combining light curves and spectral data to characterize the event's duration, variability, and energetics. Then, they went over various possible scenarios that may lead to different kinds of GRB events to determine which best fits the scenario presented by GRB 250702B.

In addition to its long duration, GRB 250702B data indicated that it had an unusually high peak energy and a minimum variability timescale (MVT) of around one second or 0.5 seconds in its rest frame. The MVT gives some indication of the mass of the "stellar engine"—meaning that it helps determine what kind of structures, like stars or black holes, are involved.

"We find a hard spectrum, subsecond variability, and high total energy, which are only known to arise from ultrarelativistic jets powered by a rapidly-spinning stellar-mass central engine. These properties and the extreme duration are together incompatible with all confirmed gamma-ray burst progenitors and nearly all models in the literature," the study authors write.

Models involving collapsars didn't work because of the ultra-long duration, as there is an upper limit on collapsar times due to the entire star "spinning apart."

The authors go on to explain away a number of possibilities: "X-ray binaries and other galactic sources are excluded by our ∼10 MeV rest-frame photons and the identification of the host galaxy in Levan and team's work. Magnetar giant flares and neutron star mergers are excluded because of insufficient durations by orders of magnitude. White dwarf mergers, carbon-oxygen collapsars, helium collapsars, and binary helium star mergers are excluded because their durations cannot reproduce the total central engine time by ∼two orders of magnitude and because each would predict a peak power at early times, in contrast to the significant delay to peak power observed in GRB 250702B."

The idea that the GRB is related to a supermassive black hole at the center of another galaxy was also ruled out. The data indicated that while GRB 250702B was in another faraway galaxy, it was not located in the galaxy's center.

Ultimately, all progenitor explanations fell through, except for one. The team found that the event was best explained by the "helium merger model," in which a black hole falls into and consumes a stripped star from the inside out, releasing energy over an extended period, then ending in a supernova. The two exist in a binary system and when a star begins to expand as it burns through its hydrogen and helium, this can offset the position of the black hole, causing it to fall into the bloated star.

"Massive stars go through a series of expansion phases that, in binary systems, can lead to a situation where the binary companion is immersed in the expanding stellar envelope. The loss of orbital angular momentum in this common envelope scenario (through friction from tidal forces or bow shocks) causes the binary orbit to shrink," the authors explain.

This leads to the long and highly energetic display of GRBs, like GRB 250702B.

"The angular momentum lost from the orbit goes into the helium star and when the black hole reaches the center of the core, this high angular momentum will cause the helium core to accrete through a disk. This disk can produce the magnetic fields required to drive jets and viscosity in the disk will drive strong winds. This will explode the star and produce a supernova, similarly to the supernova engine in collapsars," the authors write.

The group hopes to see more such events in the future to build upon this exciting new idea. New telescopes, like the Legacy Survey of Space and Time by the Vera Rubin Observatory, in combination with those in use now, may make this possible.