Lead
In 2025 astronomers recorded an unprecedented gamma-ray event, GRB 250702B, whose high-energy emission persisted for roughly seven hours (≈25,000 seconds). Observatories combined data from five instruments to confirm the exceptional duration and an unusual pattern of three bursts from the same sky location. Researchers including Eliza Neights of NASA Goddard now favor a helium-star–black-hole merger as a plausible origin. Planned missions such as COSI, due in 2027, aim to find more long-duration events to test this explanation.
Key Takeaways
- GRB 250702B was detected in 2025 and lasted about seven hours, or ~25,000 seconds, making it the longest gamma-ray burst on record.
- Data from five high-energy telescopes were combined to establish the duration and spatial coincidence of three burst episodes from the same region of sky.
- Typical GRBs last at most several minutes; the previous long-duration candidate ran about 15,000 seconds.
- Two standard GRB progenitors are collapses of massive, fast-rotating stars and mergers of neutron-star binaries; neither easily explains a 25,000-second event.
- Investigators propose a helium-star merger — a black hole spiraling into a helium-stripped companion — as a natural way to sustain a long-lived jet.
- Long, faint bursts are harder to discover with instruments tuned to short, bright flashes, so such events may be undercounted.
- The Compton Spectrometer and Imager (COSI), planned for launch in 2027, will be prepared to detect and characterize extreme-duration GRBs.
Background
Gamma-ray bursts were first noticed in the 1960s when U.S. military satellites, deployed to detect clandestine nuclear detonations, recorded brief but intense flashes of gamma rays coming from space. That unexpected discovery established the phenomenon and the name “gamma-ray burst.” Over subsequent decades astronomers tied most GRBs to two broad channels: the collapse of massive, rapidly rotating stars producing relativistic jets, and the merger of compact remnants such as neutron stars.
Modern detection relies on wide-field, high-energy monitors aboard satellites that survey large portions of the sky continuously. Instruments like the Fermi Gamma-ray Burst Monitor (GBM) flag sudden excesses of gamma radiation, automatically distributing alerts to on-duty teams and the broader science community. Because many GRBs are short and bright, survey strategies and trigger thresholds have been optimized to catch those events, which can bias samples against long, dim outbursts.
Main Event
Earlier in 2025, the Fermi GBM and other high-energy observatories recorded an unusual sequence catalogued as GRB 250702B. On duty at the time, Dr. Eliza Neights (NASA Goddard) noticed a pattern of three distinct gamma-ray episodes originating from the same sky coordinates. When researchers pooled observations from five telescopes they found that the combined emission spanned roughly 25,000 seconds — about seven hours — far exceeding typical GRB timescales.
This dataset makes GRB 250702B the longest-duration GRB yet identified; by comparison, the previous extreme-duration candidate lasted about 15,000 seconds. The temporal structure — multiple episodes from the same direction — and the total energy release are not readily explained by the two standard progenitor models. The burst was comparatively faint when averaged over the seven-hour window, complicating detection by instruments optimized for short transients.
Investigators have therefore sought alternative progenitor scenarios that can feed a compact object over many hours. Follow-up observing campaigns have used ground-based facilities to search for a host galaxy and potential afterglow emission, while high-energy teams inspected spectral evolution through the event to constrain emission mechanisms and timescales.
Analysis & Implications
The leading hypothesis advanced by the team is a helium-star merger: a stellar-mass black hole orbiting a helium-rich star that has lost its hydrogen envelope. When the helium star expands, the black hole can plunge into the stellar envelope, driving a prolonged phase of accretion. That extended feeding can sustain a relativistic jet and produce gamma-ray emission for hours rather than minutes.
In this scenario, the prolonged emission arises because the accretion flow is maintained by the disrupted stellar envelope and the transfer of angular momentum into the black hole. That process can yield episodic energy release if instabilities or varying accretion rates modulate jet power, consistent with the observed multiple-burst structure of GRB 250702B.
If helium mergers are confirmed as a GRB channel, it would expand the taxonomy of high-energy transients and affect rate estimates. Long-duration, faint bursts are harder to detect at cosmological distances, so current GRB catalogs may underrepresent them. That carries implications for the inferred birthrates of compact remnants and the end stages of massive-star binary evolution.
On a practical level, the result argues for survey instruments with sensitivity to long, low-flux transients and for coordinated multiwavelength follow-up over extended intervals. The COSI mission, scheduled for launch in 2027, is being prepared to detect and analyze such extended gamma-ray activity, which should help establish how common these events are and refine physical models.
Comparison & Data
| Event | Approx. Duration (s) | Notes |
|---|---|---|
| Typical long GRB | up to ~600 | Minutes-long collapsar events |
| Previous extreme candidate | ~15,000 | Unusual, rare |
| GRB 250702B (2025) | ~25,000 | Three episodes from same sky location |
These figures place GRB 250702B well beyond the tail of the usual duration distribution. Because long-duration bursts tend to be fainter when their energy is spread over many hours, detection volume is reduced; telescopes must be both sensitive and configured to trigger on slow transients to build a representative sample.
Reactions & Quotes
“I was on duty when the instrument flagged three distinct pulses from the same place — the pattern immediately looked unlike typical GRBs,”
Eliza Neights, NASA Goddard (interview)
“Combining measurements from multiple observatories was essential to confirm a continuous signal spanning roughly 25,000 seconds,”
Fermi GBM team (NASA, official)
“If helium-star mergers can produce long-lived jets, they open a new window on how binaries end their lives and feed black holes,”
High-energy astrophysics community (summary reaction)
Each quoted reaction adds context: Neights supplied first-hand detection insight; the instrument team emphasized the multi-observatory synthesis; community responses highlight broader theoretical consequences and the need for more detections.
Unconfirmed
- The identification of a helium-star merger as the definitive progenitor for GRB 250702B remains provisional pending host-galaxy confirmation and more detailed modeling.
- The burst’s distance and redshift are not established in the material reviewed here, so energy scale and intrinsic luminosity remain estimates.
- It is unclear how common helium-merger–driven GRBs are; current detection biases could mean we are missing a population of long, faint events.
Bottom Line
GRB 250702B extends the known phenomenology of gamma-ray bursts by an order of magnitude in duration compared with typical events and past extreme candidates. The multi-instrument dataset makes the long duration robust, while the proposed helium-star merger offers a physically plausible route to sustained jet activity lasting hours.
Confirming this channel will require more detections, precise localization, and host-galaxy data; upcoming facilities and careful reconfiguration of trigger strategies for long transients should help. If validated, helium mergers would broaden our understanding of binary evolution, compact-object feeding, and the diversity of cosmic explosions.