On July 2, 2025, a breathtaking display of nature’s wonder concert took center stage across the continental United States. NASA’s Fermi Gamma-ray Burst Monitor observed GRB 250702B, making it the longest gamma-ray burst ever observed. This exceptional outburst lasted around 25,000 seconds, roughly seven hours. It beat the previous record-holder, GRB 111209A, by a jaw-dropping 10,000 seconds. This remarkable finding has provided crucial new information about the evolution of the most massive stars. It provides important insight into how black holes form in binary systems.
The source of GRB 250702B’s brilliance has been traced back to a binary system, in which a black hole is devouring their overwhelmed companion, a bloated star. As the star expands while consuming its hydrogen and helium, it can displace the black hole’s position, leading to an eventual engulfment. This insane interplay is typical of the complex, wondrous process we call stellar evolution. It helps to illuminate the mechanics that can result in these tremendous outpourings of energy.
Arguably, GRB 250702B had the highest peak energy ever measured. It measured a minimum variability timescale (MVT) of ~1s, with suggestions as low as 0.5s in its rest frame. The whole affair unfolded in a far-flung galaxy, many parsecs from its capital. This atypical environment doubles the excitement for the surprise cosmic event.
Analysis of GRB 250702B
It took a combination of instruments to discover and observe GRB 250702B. The event was detected by NASA’s Fermi-GBM. Observing it were NASA’s Einstein Probe Wide-field X-ray Telescope and the Russian gamma-ray spectrometer Konus-Wind. The united work between these instruments has given astronomers unprecedented data, forcing them to reexamine their theories about gamma-ray bursts.
The data we’ve acquired thus far indicates that traditional progenitor models, such as white dwarf mergers and binary helium star mergers, are insufficient. Now, they can’t explain the duration and characteristics of GRB 250702B.
“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,” – phys.org
This clear exclusion of known models begs the question of what the mechanism at play with GRB 250702B is. These results are indicative of this event being a new class of gamma-ray bursts, one that is distinct from previously established classes.
Stellar Dynamics at Play
The physical mechanisms responsible for GRB 250702B are complex interactions of massive stars in binary associations. It is important to note, researchers emphasize, that underlying dynamics massive stars undergo various phases of expansion. These phases may lead to the engulfment of a binary companion by an expanding orbital stellar envelope.
“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 authors.
With this immersion, angular momentum loss proceeds because of tidal-friction, or bow-shock, tidal-force drag friction. As a result, this loss leads the orbit to decay, pushing the black hole closer to the expanding star.
The very high angular momentum that this process provides is key. It plays an important role in the accretion of the helium core by means of a disk. This accretion disk is able to create the magnetic fields required to power jets. In addition, viscosity in the disk can produce powerful winds that could trigger a supernova detonation.
“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,” – the authors.
Such a supernova engine could reproduce unexplained observable phenomena in collapsars, furthers our understanding of stellar death and rebirth in greater detail.
Implications for Future Research
The discovery of GRB 250702B leads to significant opportunities for future studies within astrophysics, especially related to the exploration of gamma-ray bursts and their progenitors. Beyond the format, this event has special features. We need to reconsider our current paradigms and concepts of massive stars and how they interact with black holes.
“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,” – Eliza Neights et al.
As scientists continue to analyze data from GRB 250702B, they will undoubtedly uncover more about the nature of gamma-ray bursts and their connection to black holes and stellar evolution. This uncommon, but spectacular, occurrence deepens our knowledge of the universe we inhabit. It gives us a hint of the amazing cosmic phenomena we haven’t found yet.