It lasted a thousand times longer than the average gamma-ray burst.
On July 2, 2025, the NASA Fermi Gamma-ray Space Telescope picked up an event that would shake decades of astrophysical knowledge. Known as GRB 250702B, it wasn’t one brief flash of intense high-energy light, but a series of intense explosions over the course of almost 24 hours. “This event is unlike any other seen in 50 years of GRB observations,” said Antonio Martin-Carrillo of University College Dublin, co-lead author of the paper in The Astrophysical Journal Letters.
Gamma-ray bursts (GRBs) are the cosmos’ brightest explosions, spanning from milliseconds to minutes. They are believed to occur when gigantic stars implode into black holes collapsars or when black holes shred stars apart during tidal disruption events (TDEs). In both instances, the destruction is fatal; the source is annihilated. “Gamma-ray bursts never repeat since the event that produces them is catastrophic,” Martin-Carrillo said. But GRB 250702B had emitted three separate gamma-ray flares on consecutive hours, and X-ray activity had been detected by the Einstein Probe just about a full day prior to this.
The mystery grew when the European Southern Observatory’s Very Large Telescope (VLT) and NASA’s Hubble Space Telescope identified the source in a galaxy a few billion light-years away. The extragalactic origin implied the intrinsic energy of the burst was much higher than initially thought. “What we found was considerably more exciting: the fact that this object is extragalactic means that it is considerably more powerful,” Martin-Carrillo explained.
The sheer timescale “100 to 1000 times longer than most GRBs,” co-lead author Andrew Levan of Radboud University says is prompting astronomers to think about exotic mechanisms. The top idea is tidal disruption of a white dwarf by an intermediate-mass black hole (IMBH), a long-hypothesized but seldom proven class of black holes with masses between stellar and supermassive sizes. In that case, the orbit of the white dwarf might be causing periodic bursts as it gets ripped apart, possibly accounting for the seeming regularity in GRB 250702B’s emission periods.
Its periodicity is especially tantalizing. Scrutiny of the Fermi observations found that the third gamma-ray burst happened at an integer multiple of the time difference between the first two, suggesting a cyclical engine more than a one-time explosive ejection. Although some relativistic TDEs can emit high-energy radiation for hours or days, GRB 250702B’s offset from its host galaxy’s nucleus eliminates the more typical supermassive black hole type.
Standard collapsar simulations also struggle with the data. Core collapse in a massive star should produce a burst that lasts for seconds, accompanied by an afterglow as relativistic jet interacts with ambient gas. The multiwavelength afterglow of GRB 250702B, seen in X-ray, optical, and infrared, conforms to some parts of established afterglow physics, but the morphology of prompt emission faint then becoming bright is at odds with predictions. The event’s morphology and energy correlations “further challenge the interpretation of GRB 250702B as a core-collapse event,” the authors wrote.
Detection and analysis relied on a network of instruments. Fermi’s Gamma-ray Burst Monitor provided the initial high-energy light curves, while the Einstein Probe’s Wide-field X-ray Telescope captured the earlier X-ray phase. The VLT’s HAWK-I camera and X-shooter spectrograph further determined the source location and redshift, while Hubble imaging showed a convoluted host galaxy morphology, perhaps with a dust lane cutting across the disk. Such multi-instrument methodology is reminiscent of methodology employed in previous paradigm-changing events, such as GRB 221009A, where space- and ground-based observatories’ rapid coordination revealed unforeseen jet morphologies and emission components.
GRB 250702B’s energy release is mind-boggling. Even a “weaker” measured flux, when adjusted for its extragalactic distance, suggests an isotropic-equivalent energy release equal to the Sun’s total lifetime output over a single day of episodic outrage. The physics by which such a central engine can persist so long through fallback accretion, magnetic activity, or orbital modulations in a TDE is still a problem.
For now, the research team continues to monitor the fading afterglow with the James Webb Space Telescope and the VLT, hoping to constrain the progenitor’s nature. As Levan explained, “If this is a massive star, it is a collapse unlike anything we have ever witnessed before.” The possibility that it represents the first confirmed detection of a white dwarf–IMBH tidal disruption only adds to the stakes, providing a glimpse into a regime of black hole physics previously inaccessible to direct observation.
