When space telescopes detected a prolonged burst of high-energy radiation earlier this year, astronomers knew they were witnessing a rare cosmic event. The phenomenon, known as GRB 250702B, was not just any gamma-ray burst; it was the longest-lasting one ever recorded, persisting for nearly seven hours.
Gamma-ray bursts (GRBs) are the universe’s most powerful explosions, typically fading within minutes. However, GRB 250702B outlasted all previous records, including the former record-holder, GRB 111209A, by nearly three hours. This extraordinary event has prompted scientists to rethink their understanding of these cosmic occurrences.
A Burst Unlike Any Other
The event was first detected on July 2, 2025, when NASA’s Fermi Gamma-ray Burst Monitor identified unusual signals lasting over three hours. Other observatories, including the Konus-Wind detector, Japan’s Einstein Probe, and the Psyche-GRNS instrument, confirmed the extended emission from the same source.
By synthesizing data from the InterPlanetary Network, researchers reconstructed the event’s profile, recording a duration of approximately 25,000 seconds, or about seven hours. “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,” the study’s authors explained.
This was not the typical explosion expected when a giant star forms a black hole, known as a “collapsar.” The evidence defied existing models and hypotheses.
Finding the Source
An international team of over 50 astrophysicists explored various possibilities to explain GRB 250702B. They considered whether it was a magnetar flare, a neutron-star merger, or a supermassive black hole at a galaxy’s center. Each possibility was systematically ruled out.
The energy outputs were too high for a magnetar or binary merger, and the event’s timescale exceeded what collapsars could produce. Additionally, its location, away from the host galaxy’s nucleus, dismissed the possibility of an active galactic nucleus.
The only viable 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 each other until one collapses into a black hole. The other star expands, and the black hole moves towards the center, trapped within the expanding stellar envelope. The friction tightens the orbit until the black hole penetrates the star’s helium core.
A powerful accretion disk forms around the black hole, propelling material inward and ejecting twin jets at near-light speed. These jets last tens of thousands of seconds, producing the gamma rays observed on Earth.
“The angular momentum taken out of the orbit is transferred to the helium star, and as the black hole is making its way to the center of the core, the high angular momentum will propel the helium core to accrete through a disk,” the authors explained.
Magnetic fields and turbulence within the disk generate enormous energy, igniting an explosion that resembles—and can outlast—a supernova.
Testing the Theory
Simulations involving black holes with masses around two solar masses and helium stars between 32 to 60 solar masses replicated the light curves of GRB 250702B. The Blandford–Znajek process, a mechanism for extracting energy from a spinning black hole, mirrored the burst’s steady rise and decline in intensity.
Even minute details, such as photon energy shifts and sub-second flickering, aligned with 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 originated from a single, continuous event.
The Supernova That Wasn’t
Following the initial burst, astronomers revisited the site with the James Webb Space Telescope, anticipating an incandescent supernova 25 days later. Instead, they found nothing—no afterglow or transient light.
This absence may bolster the helium-merger theory. Simulations suggest that after a black hole reaches five solar masses, conditions in the accretion disk inhibit the formation of nickel-56, the radioactive isotope that illuminates supernovae. Any subsequent explosion would be too weak to outshine the host galaxy’s dust.
The Team Behind the Discovery
The success of this project was due to international collaboration. Eliza Neights led the background fitting and spectral analysis of Fermi GBM data. Eric Burns coordinated the collaboration and assessed the event’s energy budget. Chris Fryer conducted helium-merger physics simulations, and Dmitry Svinkin compared Konus-Wind data with previous gamma-ray bursts.
The study received support from NASA, the United States–Israel Binational Science Foundation, and international partners who provided pre-publication data for expedited review.
A New Window Into the Cosmos
The detection of GRB 250702B opens a new chapter in high-energy astronomy. It offers the strongest evidence yet that some ultra-long gamma-ray bursts result from black hole and massive star mergers, not the collapse of individual stars.
This realization reshapes scientific understanding of stellar death, jet formation, and gravitational-wave sources. The merger of a black hole with a star may link two previously separate phenomena—gamma-ray bursts and the spacetime ripples detected by observatories like LIGO and Virgo.
Future missions, such as 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 observation, scientists anticipate discovering more ultra-long GRBs and unraveling the full life cycles of the stars that fuel them.
Practical Implications of the Research
Decoding GRB 250702B not only sets a new record but also advances our understanding of black hole formation and binary star system evolution.
Such discoveries could refine supernova models and improve predictions of gravitational-wave signals, helping scientists pinpoint the origins of immense 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.
Research findings are available online in the journal arXiv.
Related Stories
- Astronomers capture first-ever image of two black holes orbiting each other
- Black hole stars: Giant stars may hide black holes at their core
- A wormhole from another universe? Scientists revisit the puzzling black hole GW190521
Like these kinds of feel-good stories? Get The Brighter Side of News’ newsletter.
