When space telescopes detected an unprecedented surge of high-energy radiation earlier this year, astronomers knew they were observing something extraordinary. The cosmic event, named GRB 250702B, was not a typical gamma-ray burst (GRB); it was the longest-lasting one ever recorded, persisting for nearly seven hours.
Gamma-ray bursts are among the universe’s most cataclysmic occurrences, typically fading within minutes or even seconds. However, GRB 250702B outlasted all previous records, surpassing the former record-holder, GRB 111209A, by almost three hours. This extraordinary event has prompted scientists to reconsider the mechanisms behind such cosmic phenomena.
A Burst Unlike Any Other
The event was first observed on July 2, 2025, when NASA’s Fermi Gamma-ray Burst Monitor identified unusual signals that lasted for more than three hours. Subsequent data from other observatories, including the Konus-Wind detector, Japan’s Einstein Probe, and the Psyche-GRNS instrument, confirmed that the extended emission originated from the same source.
By integrating signals from the InterPlanetary Network, researchers reconstructed the complete profile of the event, recording a duration of approximately 25,000 seconds, or about seven hours. The study’s authors noted,
“We see a hard spectrum, subsecond variability, and high total energy, which are only known to come from ultrarelativistic jets powered by a millisecond-stable stellar-mass central engine.”
This was not the typical explosion expected when a giant star collapses into a black hole, known as a “collapsar.” The evidence defied existing models, leading scientists to explore new explanations.
Finding the Source
To understand GRB 250702B, an international team of over 50 astrophysicists examined nearly every possible scenario. Was it a magnetar flare, a neutron-star merger, or a supermassive black hole at a galaxy’s center? Each theory was systematically ruled out.
The energy outputs were too high for a magnetar or binary merger, and the timescale was far beyond what collapsars could sustain before their stars lose momentum. Additionally, the event’s location, away from the nucleus of its host galaxy, eliminated the possibility of an active galactic nucleus.
The remaining explanation was a rare and violent cosmic interaction known as a helium-merger event—a black hole merging with the helium core of a companion star.
The Helium Merger Model
In this model, two massive stars orbit one another until one collapses into a black hole. The companion star expands, and the black hole spirals inward, becoming trapped within the star’s expanding envelope. This friction tightens the orbit until the black hole penetrates the star’s helium core.
At this point, a powerful accretion disk forms around the black hole, propelling material inward and ejecting twin jets at nearly the speed of light. These jets, lasting tens of thousands of seconds, produce the gamma rays observed on Earth.
The authors explained,
“The angular momentum taken out of the orbit is transferred to the helium star, and as the black hole moves to the core’s center, the high angular momentum propels the helium core to accrete through a disk.”
Magnetic fields and turbulence within the disk generate enormous energy, igniting an explosion that can resemble—and even outlast—a supernova.
Testing the Theory
Simulations involving black holes with masses around two solar masses and helium stars weighing between 32 and 60 solar masses replicated the light curves observed in GRB 250702B. The Blandford–Znajek process, which extracts energy from a spinning black hole, mirrored the burst’s steady rise and gradual decline in intensity.
Even minute details, such as shifts in photon energy and sub-second flickering, matched observations from Fermi’s NaI and BGO detectors. These findings, along with extended detections by MAXI and Psyche-GRNS, confirmed that the long-lived emission was a single continuous event rather than a series of unrelated flashes.
The Supernova That Wasn’t
Following the initial fireworks, astronomers used the James Webb Space Telescope for a second look, expecting to find a supernova at the burst’s site about 25 days later. Instead, they found nothing—no afterglow or characteristic transient light.
This absence might actually support the helium-merger hypothesis. Simulations suggest that once a black hole reaches five solar masses, conditions in the accretion disk prevent the formation of nickel-56, the radioactive isotope that illuminates supernovae. Any resulting explosion would be too weak to outshine the host galaxy’s dusty light.
The Team Behind the Discovery
The project was a triumph of international collaboration. Eliza Neights led the background fitting and spectral analysis of the Fermi GBM data. Eric Burns coordinated the collaboration and assessed the event’s energy budget. Chris Fryer conducted helium-merger physics simulations, while Dmitry Svinkin analyzed Konus-Wind data to compare GRB 250702B with previous gamma-ray bursts.
The study was supported by NASA, the United States–Israel Binational Science Foundation, and international collaborators who provided pre-publication data for an expedited review of the results.
A New Window Into the Cosmos
The detection of GRB 250702B opens a new chapter in high-energy astronomy. It provides the strongest evidence yet that some ultra-long gamma-ray bursts result from black hole mergers with massive stars, rather than the collapse of individual stars.
This realization reshapes our understanding of stellar death, jet formation, and potential gravitational-wave sources. The merger of a black hole with a star could link two previously separate phenomena—gamma-ray bursts and the spacetime ripples detected by observatories like LIGO and Virgo.
Future missions, including the Legacy Survey of Space and Time by the Vera Rubin Observatory and the upcoming Compton Spectrometer and Imager, will enhance the detection of these exotic cosmic events. With improved coverage and extended observations, scientists anticipate discovering more ultra-long GRBs and unraveling the full life histories of the stars that fuel them.
Practical Implications of the Research
Decoding GRB 250702B not only sets a new record—it provides insights into black hole formation and binary star system evolution. Such discoveries could refine supernova models and improve predictions of gravitational-wave signals, helping scientists trace the origins of colossal cosmic collisions.
By linking GRBs with black-hole mergers, researchers move closer to mapping how elements, radiation, and even life’s building blocks are distributed throughout the universe. The research findings are available online in the journal arXiv.
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