On July 2, 2025, a gamma-ray signal lasting roughly seven hours lit up detectors aboard NASA’s Fermi Gamma-ray Space Telescope and China’s Einstein Probe, triggering a cascade of follow-up observations from some of the most powerful telescopes ever built. Designated GRB 250702B, the event defied the typical behavior of gamma-ray bursts, which usually fade within seconds or minutes. Two peer-reviewed papers have since characterized it as an ultra-long, repeating transient with no clean precedent, and competing theories now pit conventional stellar-collapse models against a stranger possibility, a black hole plunging into the heart of a companion star.
A Seven-Hour Signal That Broke the Mold
Gamma-ray bursts rank among the most energetic explosions in the known universe, yet even the longest ones rarely persist beyond a few minutes. GRB 250702B shattered that expectation. The Einstein Probe Wide-field X-ray Telescope detected an X-ray transient labeled EP250702a during a timing window spanning multiple observations on July 2, with its spectrum fitting an absorbed power-law model and flux measured in the 0.5 to 4 keV band. The Einstein Probe team explicitly linked EP250702a to a cluster of Fermi Gamma-ray Burst Monitor triggers: GRB 250702B, C, D, and E. That string of designations reflects the fact that Fermi’s onboard software treated each burst of activity as a separate event, even though they all originated from the same patch of sky.
The multi-trigger record is itself telling. Fermi’s GBM issued final real-time localization circulars for each trigger, complete with precise trigger times, right ascension and declination coordinates, and statistical uncertainties. That the instrument fired four distinct alerts from one source over hours, rather than registering a single smooth emission curve, pointed to repeating pulses rather than a steady glow. A peer-reviewed analysis published in The Astrophysical Journal Letters formally classified the event as an ultra-long transient, synthesizing detections from both space-based and ground-based instruments to build the quantitative case that GRB 250702B sits outside established categories of stellar explosions.
A Fleet of Telescopes Chased the Afterglow
Within days of the initial detection, major observatories pivoted their schedules to track the fading signal. NuSTAR conducted a Target-of-Opportunity X-ray observation of EP250702a and GRB 250702B, coordinating its pointing with Swift observations from the X-Ray Telescope to capture the afterglow across overlapping energy ranges. That kind of rapid mobilization, where two independent X-ray satellites align on the same target, is reserved for events that researchers judge to be scientifically exceptional. The coordinated campaign extended the observational record well beyond the initial gamma-ray triggers, allowing teams to track how the source’s brightness decayed over time and to compare the evolution of the X-ray emission with the earlier high-energy flashes.
The Hubble Space Telescope joined the effort on July 15, imaging the field in optical and infrared wavelengths. Those Hubble images served a specific purpose: confirming the presence and position of a host galaxy, which anchored GRB 250702B as an extragalactic event rather than something closer to home. Weeks later, the Chandra X-ray Observatory picked up a late-time counterpart for GRB 250702B, reporting a flux estimate in the 0.3 to 10 keV band tied to an assumed spectral model. The detection of residual X-ray emission that far after the original burst placed tight constraints on how much energy the source released and how the surrounding material was cooling. Taken together, the ALMA submillimeter detection noted on the GCN event page, the NuSTAR and Swift X-ray windows, Hubble’s optical imaging, and Chandra’s late-time measurement created one of the most densely sampled follow-up campaigns for any gamma-ray burst in recent memory.
A Black Hole Falling Into a Star
Two independent research teams have now published peer-reviewed interpretations of what produced the signal, and they converge on a scenario that has no established analog. A study in Monthly Notices of the Royal Astronomical Society frames GRB 250702B as the product of a black hole-star encounter, offering detailed gamma-ray analysis that includes duration determination, energetics context, and multi-instrument comparisons. The physical picture starts with a stellar-mass black hole and a large companion star locked in a binary orbit. As the star exhausts its hydrogen fuel and its outer layers expand, the black hole spirals inward through the envelope and ultimately approaches the helium core. Each pass through dense stellar material could trigger a fresh burst of accretion and jet activity, which would explain the repeating pulse structure that Fermi recorded as four separate triggers.
That model carries a provocative implication. If black holes routinely spiral into companion stars, the process should produce electromagnetic signatures that existing surveys have simply never recognized. The seven-hour duration and episodic flaring of GRB 250702B may represent the first clear detection of a phenomenon that has been hiding in plain sight, misclassified as noise or folded into broader categories of variable X-ray sources. The repeating nature of the signal suggests that the in-spiraling black hole may have interacted multiple times with dense layers inside the star before either merging with the core or disrupting the stellar envelope, with each interaction launching a jet that briefly pierced the surrounding gas and produced a gamma-ray flash.
Challenging Standard Gamma-Ray Burst Categories
Standard gamma-ray bursts are typically divided into “short” and “long” classes, with durations of less than two seconds and more than two seconds, respectively, and are usually tied to either compact-object mergers or the collapse of massive stars. Ultra-long events lasting thousands of seconds have been reported before, but they often show a smoother decay and can sometimes be explained by unusually extended stellar envelopes or magnetar-powered engines. GRB 250702B, by contrast, combined an extreme duration with discrete, well-separated pulses, forcing theorists to revisit whether the existing taxonomy can accommodate such behavior. The detailed timing and spectral evolution recorded across the Fermi and Einstein Probe detections indicate a central engine that turned on and off repeatedly rather than fading monotonically, which is difficult to reconcile with a single, one-off collapse.
In this light, the black hole–in-spiral model offers a way to bridge the gap between burst-like and persistent emission. As the compact object plows through the star’s envelope, it can accrete material intermittently, launching jets each time accretion ramps up and then shutting down as local fuel is exhausted. The intervals between pulses could encode information about the structure of the star’s interior, such as the density contrast between different layers, while the overall duration reflects how long it takes the black hole to traverse the envelope. If future events of this type are identified, comparing their temporal patterns could effectively turn them into probes of stellar interiors that would otherwise be inaccessible.
A Glimpse of a New Class of Cosmic Catastrophes
For now, GRB 250702B stands as a single, striking data point, but its discovery is already reshaping how observers comb the sky for transients. The fact that Fermi’s software initially split the emission into four separate bursts hints that other ultra-long, repeating events might have been fragmented in archival data and never stitched together into a coherent picture. With the new theoretical framework in hand, teams can re-examine past gamma-ray and X-ray catalogs for patterns of clustered triggers from the same location over several hours, potentially revealing additional candidates. The dense follow-up campaign for GRB 250702B also sets a template for how to respond when another such signal appears, emphasizing rapid coordination across space- and ground-based facilities.
Whether or not black hole–star mergers turn out to be common, the seven-hour fireworks show of GRB 250702B has expanded the known repertoire of how massive stars can die and how compact objects can interact with their environments. The event underscores the value of wide-field monitors that can catch rare, long-lived transients, and of rapid-response networks that can mobilize telescopes across the spectrum to capture every phase of the outburst. As new missions with greater sensitivity come online, astronomers expect that what now looks like a singular oddity may soon be joined by a growing catalog of similar catastrophes, each one offering fresh clues about the extreme physics that govern the universe’s most violent events.
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*This article was researched with the help of AI, with human editors creating the final content.
