GRB 250702B: Discovery of a Gamma-Ray Burst from a Black Hole Falling into a Star

TL;DR

GRB 250702B is the longest gamma-ray burst to date, with prompt emission lasting $ rightarrow$25{,}000–30{,}000 s, subsecond variability, and a hard spectrum. By integrating data from multiple gamma-ray instruments, the study derives an ultrarelativistic jet (Γ$_0$ in the tens to ~80 range) powered by a long-lived central engine, and finds $E_{ m iso}\gtrsim 1.4\times 10^{54}$ erg with high $E_p$, placing the burst as an outlier to standard Amati/Yonetoku relations. The authors argue that traditional progenitors cannot explain the duration, energetics, and timing, and present helium-star mergers (a stellar-mass black hole accreting from a stripped helium star) as the natural origin, compatible with jet energetics, lack of a bright associated supernova, and population expectations tied to star formation. This work strengthens the case for helium-star mergers as a viable channel for ultra-long GRBs and outlines concrete observational tests (host environments, nickel yields, and late-time transients) for future validation.

Abstract

Gamma-ray bursts are the most luminous electromagnetic events in the universe. Their prompt gamma-ray emission has typical durations between a fraction of a second and several minutes. A rare subset of these events have durations in excess of a thousand seconds, referred to as ultra-long gamma-ray bursts. Here, we report the discovery of the longest gamma-ray burst ever seen with a ~25,000 s gamma-ray duration, GRB 250702B, and characterize this event using data from four instruments in the InterPlanetary Network and the Monitor of All-sky X-ray Image. We find a hard spectrum, subsecond variability, and high total energy, which are only known to arise from ultrarelativistic jets powered by a rapidly-spinning stellar-mass central engine. These properties and the extreme duration are together incompatible with all confirmed gamma-ray burst progenitors and nearly all models in the literature. This burst is naturally explained with the helium merger model, where a field binary ends when a black hole falls into a stripped star and proceeds to consume and explode it from within. Under this paradigm, GRB 250702B adds to the growing evidence that helium stars expand and that some ultra-long GRBs have similar evolutionary pathways as collapsars, stellar-mass gravitational wave sources, and potentially rare types of supernovae.

Figures (18)

• Figure 1: The combined, background-subtracted gamma-ray lightcurve of GRB 250702B. For Konus-Wind, we show counts in the 75--315 keV energy range, for Psyche-GRNS the 30--230 keV energy range, and for Fermi-GBM 50-500 keV for NaI and 400--2,000 keV for BGO detectors. Detector lightcurves are scaled and Psyche is shifted by 1,000 s (to account for light travel time) for visualization. Intervals where the data for a given instrument are not useful are removed (i.e. times with unrelated GRBs, Earth-occluded times, etc.). Top: Lightcurves for Psyche-GRNS with 600 s temporal resolution and Konus-Wind and Fermi-GBM both at 120 s temporal resolution. The prompt gamma-ray emission from GRB 250702B begins at least by T0+46,074 s based on the rapid rise and lasts at least until T0+71,600 s based on the last significant flare, as confirmed by Swift-BAT emission from the source. The BAT non-detections show the burst has quiescent intervals. MAXI information confirms these results. Bottom: A view of the brightest region of gamma-ray emission with Fermi-GBM and Konus-Wind data shown at 30 s resolution. Also shown are the selection intervals for detailed analyses.
• Figure 2: GRB 250702B in the context of Fermi-GBM GRBs. Left: The duration and peak energy in the peak flux interval. Right: The observed MVT as a function of 64 ms peak photon flux. Also shown is an approximate lower limit on the known MVT from TDEs, based on Swift J1644+57, detailed in Appendix \ref{['app:tde']}. While the duration of GRB 250702B is an extreme outlier and the $E_p$ is unusually high, the MVT and peak flux are typical values.
• Figure 3: GRB 250702B intrinsic energetics compared to the broader population of GRBs from burns2023grb. We also highlight the second and third longest bursts. These events have high $E_{\rm iso}$ and low $L_{\rm iso}$ values that fall within the known distributions, but have extreme ratios of these two values incompatible with the broader population. The dashed line in the left two plots correspond to the approximate Fermi-GBM detection threshold.
• Figure 4: The Konus-Wind Amati (left) and Yonetoku (right) relations with 68% and 90% confidence intervals, using data from tsvetkova2017konustsvetkova2021konus. $E_{p,i}$ is the peak energy from the time-integrated interval, while $E_{p,p}$ is the peak energy from the peak flux interval. GRB 250702B is marked with a star, while other ultra-long GRBs (defined as durations above 1,000 s) with measured redshift are shown with white shapes.
• Figure 5: Specific angular momentum versus mass coordinate for a single star with moderate dynamo models locking different burning layers, tidally-locked Helium and CO star collapsars fryer2019understandingfryer2025explaining, and our helium star merger model. The two plots show the results for the cores produced by two different 1/10th solar, stellar models (32 and 60 M$_\odot$ zero-age main sequence stars) at CO depletion. A powerful jet requires an accretion disk at $\sim$100 km, i.e. viable GRB models must exceed the red line above.