Record gamma-ray burst appears to be caused by black hole swallowed up by bloated star

On July 2, 2025, NASA’s Fermi Gamma Burst Monitor (Fermi-GBM) captured approximately three hours of signals that appeared to come from the same source. When scientists compiled this data with signals captured by several other instruments, such as the Einstein Probe (EP) wide-field X-ray telescope and the Russian Konus-Wind gamma-ray spectrometer, they discovered that they were dealing with the longest gamma-ray burst (GRB) ever recorded. Lasting about 25,000 seconds (about seven hours), the GRB event that scientists call “GRB 250702B” beat the previous record holder, GRB 111209A, by 10,000 seconds.

Most GRBs detected in the past have only lasted from less than a second to a few minutes. Thus, such long bursts of powerful gamma radiation in space are rare. However, these ultra-long gamma-ray bursts do occur, and for the most part, scientists have found reasonable explanations. Most long-lived GRBs have been linked to the collapse of massive stars, called collapses, while short-lived GRBs are linked to the merger of neutron stars. But when scientists calculated the various properties of GRB 250702B, it didn’t quite fit the mold of previous explanations about GRB progenitors.

In a new arXiv preprint, a group of more than 50 scientists came together to find out how, why, and where GRB 250702B came to be. In this paper, the team analyzed all available data, combining light curves and spectral data to characterize the duration, variability and energy of the event. Next, they looked at various possible scenarios that could lead to different types of GRB events to determine which one best fits the scenario presented by GRB 250702B.

In addition to its long duration, data from GRB 250702B indicated that it had an unusually high peak energy and a minimum variability time scale (MVT) of approximately one second or 0.5 seconds in its resting phase. The MVT gives some indication of the mass of the “stellar engine,” meaning it helps determine what types of structures, like stars or black holes, are involved.

“We find a harsh spectrum, sub-second variability, and high total energy, which are only known to originate from ultrarelativistic jets powered by a rapidly rotating stellar mass central engine. These properties and the extreme duration are together inconsistent with all confirmed gamma-ray burst progenitors and almost all models in the literature,” the study authors write.

Models involving collapses have not worked due to their ultra-long duration, as there is an upper limit on collapse times due to the entire star “breaking apart”.

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

The idea that the GRB is linked to a supermassive black hole at the center of another galaxy has also been ruled out. The data indicated that although GRB 250702B was in another distant galaxy, it was not located in the center of the galaxy.

Ultimately, all but one of the ancestors’ explanations failed. The team found that the event was best explained by the “helium fusion model”, in which a black hole falls and consumes a stripped star from the inside out, releasing energy over an extended period, then ending as a supernova. Both exist in a binary system and when a star begins to expand by burning its hydrogen and helium, this can shift the position of the black hole, causing it to fall into the bloated star.

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

This leads to the long and very energetic display of GRBs, such as GRB 250702B.

“The lost angular momentum 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 needed to drive the jets and the viscosity in the disk will drive strong winds. This will cause the star to explode and produce a supernova, similar to the supernova engine in collapses,” the authors write.

The group hopes to see more events like this in the future to build on this exciting new idea. New telescopes, such as the Vera Rubin Observatory’s Legacy Survey of Space and Time, in combination with those currently in use, could make this possible.

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