Black hole devoured a star from within, producing the longest gamma-ray burst on record

On July 2, 2025, NASA's Fermi Gamma-ray Burst Monitor detected an extraordinary event: GRB 250702B, a gamma‑ray burst whose emission continued for about 25,000 seconds, roughly seven hours. A team of more than fifty researchers analyzed the data and published their results on the arXiv preprint server, concluding that the burst cannot be explained by any previously confirmed progenitor model.

Key observations

The burst showed several extreme properties:

• Duration of about 25,000 seconds, far longer than the previous record of roughly 15,000 seconds.
• A very hard spectrum and high peak energy, with rest-frame photons exceeding 10 MeV.
• Subsecond variability combined with high total emitted energy, characteristics normally associated with ultrarelativistic jets from a compact, rapidly spinning central engine.

Why standard models fail

The research team systematically evaluated known gamma‑ray burst progenitors and found them inconsistent with the observations:

The helium merger explanation

To account for the extreme duration, spectral hardness, and rapid variability, the team proposes a helium merger scenario. In this model, a binary system contains a stellar‑mass black hole and a companion star that evolves and expands. As the companion grows, it engulfs the black hole. The black hole then spirals inward through the companion's envelope, losing orbital energy via friction and tidal interactions until it reaches the dense helium core.

When the black hole reaches the core, the system's high angular momentum drives accretion through a disk rather than direct infall. That disk can produce strong magnetic fields and power ultrarelativistic jets while viscous processes drive powerful winds. The combination of jets and an accompanying supernova‑like explosion can produce prolonged, high‑energy gamma‑ray emission consistent with the observations of GRB 250702B.

Implications

The helium merger model links binary evolution, unusual supernovae, and long‑duration gamma‑ray bursts in a single framework. It also suggests potential connections between such events and gravitational‑wave sources, because the dynamics involve compact objects interacting deeply with stellar companions.

Data and publication