GRB 250702B: Extreme Ultra-long GRB

• GRB 250702B is the longest-duration gamma-ray burst recorded, exhibiting over 25,000 s of prompt emission with complex, multi-episodic gamma-ray and soft X-ray activity.
• Its extreme energetics (E₍γ,iso₎ ≳ 2.2×10⁵⁴ erg) and ultra-narrow jet (≈0.5° opening angle) challenge conventional massive-star collapse and tidal disruption models.
• The event’s host galaxy is massive, dusty, and morphologically disturbed, with a significant offset from the galactic center, highlighting the role of unique environments in GRB production.

GRB 250702B is the longest-duration gamma-ray burst (GRB) ever detected, with a total observed prompt emission period exceeding 25,000 seconds and additional, earlier soft X-ray activity. Its unprecedented duration, complex multi-episodic gamma-ray structure, and atypical environment challenge all previously established models of ultra-long GRB production and have led to a reevaluation of massive-star collapse, tidal-disruption, and micro-tidal disruption scenarios. The event's host galaxy is massive (M_* ≳ 10^{11} M_⊙), dusty (A_V ≳ 3 mag), and morphologically disturbed, and the source is offset several kiloparsecs from the galactic center. No spectroscopically luminous supernova has been securely identified, and the isotropic-equivalent energy is among the very highest ever measured. The event is exceptionally rare, with inferred volumetric rates orders of magnitude below typical long GRBs or core-collapse supernovae.

1. Observational Properties and Key Measurements

The prompt emission of GRB 250702B was first captured as a sequence of discrete, extraordinarily long-lasting gamma-ray episodes by Fermi-GBM, Konus-Wind, Swift-BAT, Psyche-GRNS, and MAXI, with multi-wavelength follow-up by JWST/NIRSpec and NIRCam, HST, VLT, MeerKAT, and other observatories (Beniamini et al., 26 Sep 2025, Neights et al., 26 Sep 2025, Sears et al., 16 Jun 2026, Gompertz et al., 26 Sep 2025, Granot et al., 16 Dec 2025). Four major energetic gamma-ray emission episodes were identified, spaced quasi-periodically over a ~4 hour window, with a ≈50 s gamma-ray precursor occurring ≈25 hours prior (Zhang et al., 30 Sep 2025, Levan et al., 18 Jul 2025).

Key measured properties:

Parameter	Value / Range	Instrument/Analysis
$T_{90}$ (prompt)	≳25,000 s	Fermi/Konus–Wind/etc.
Earliest X-ray precursor	0.5–1 day before main burst (γ-ray quiet)	Einstein Probe
Energetics ($E_{\gamma,\mathrm{iso}}$)	≳2.2×10^{54} erg	JWST NIRSpec, Konus
Minimum variability timescale	0.5–1 s (rest frame)	Fermi-GBM, wavelets
Redshift	$z=1.036\pm0.004$	JWST/NIRSpec
Host offset	5.5–5.7 kpc from galactic center	JWST/HST imaging
Host stellar mass	$\log(M_*/M_\odot) = 11.0$–$11.6$	JWST, Prospector/Cigale
Dust extinction ($A_V$)	$2.8$–$6.2$ mag; $A_V$(afterglow sightline) up to $5.8$ mag	Prospector, NIRSpec
Supernova constraint	No SN as bright as Type Ic-BL SN 2023lcr	JWST NIRSpec
Afterglow decay slope ($E_{\gamma,\mathrm{iso}}$0)	$E_{\gamma,\mathrm{iso}}$1–$E_{\gamma,\mathrm{iso}}$2 (X-ray/optical/radio)	Swift/NuSTAR/Chandra
Volumetric rate	≈$E_{\gamma,\mathrm{iso}}$3 yr$E_{\gamma,\mathrm{iso}}$4 Gpc$E_{\gamma,\mathrm{iso}}$5 (after beaming corr.)	(Gompertz et al., 26 Sep 2025)

The extremely high $E_{\gamma,\mathrm{iso}}$6 and ultra-narrow jet opening angle ($E_{\gamma,\mathrm{iso}}$7) set GRB 250702B apart from ordinary long GRBs (Gompertz et al., 26 Sep 2025, Sears et al., 16 Jun 2026).

2. Prompt and Afterglow Multi-wavelength Phenomena

The prompt light curve is defined by four gamma-ray emission episodes, separated by 2825–4000 s intervals, and a precursor in soft X-rays/gamma-rays ≈1 day before (Zhang et al., 30 Sep 2025, Levan et al., 18 Jul 2025). Each episode features high-energy photons ($E_{\gamma,\mathrm{iso}}$8 up to several MeV) and sub-second scale variability, indicating compact and relativistic emission regions ($E_{\gamma,\mathrm{iso}}$9) (Neights et al., 26 Sep 2025, Gompertz et al., 26 Sep 2025).

The long-lived afterglow is well-modeled as forward-shock emission in both wind-like ($z=1.036\pm0.004$0) and Bondi-density profiles (late afterglow: $z=1.036\pm0.004$1) (Sears et al., 16 Jun 2026, Granot et al., 16 Dec 2025). The X-ray afterglow decays as a single power law ($z=1.036\pm0.004$2), and late-time Chandra and NIRCam imaging show no SN-like transient, though the heavy dust attenuation limits strong constraints except for the most luminous SNe (Sears et al., 16 Jun 2026, Gompertz et al., 26 Sep 2025). The radio and NIR afterglows similarly fit synchrotron shock models with high $z=1.036\pm0.004$3 and $z=1.036\pm0.004$4, and large kinetic energies $z=1.036\pm0.004$5 erg (O'Connor et al., 26 Sep 2025, Granot et al., 16 Dec 2025).

A subset of late JWST/NIRCam photometry shows marginal (3$z=1.036\pm0.004$6) detections in F150W/F200W indicative of a late-time IR plateau or flattening, consistent with either a jetted TDE plateau or a supernova atop an afterglow, though not statistically decisive (Sears et al., 16 Jun 2026).

3. Host Galaxy Characteristics and Environment

The host of GRB 250702B is extraordinary compared to the general GRB and star-forming galaxy population. JWST/NIRSpec and SED fits yield:

• $z=1.036\pm0.004$7 (from Pa$z=1.036\pm0.004$8, H$z=1.036\pm0.004$9, Br$\log(M_*/M_\odot) = 11.0$0)
• Stellar mass $\log(M_*/M_\odot) = 11.0$1–$\log(M_*/M_\odot) = 11.0$2 (Sears et al., 16 Jun 2026, Gompertz et al., 26 Sep 2025)
• Nebular $\log(M_*/M_\odot) = 11.0$3(lines)$\log(M_*/M_\odot) = 11.0$4--$\log(M_*/M_\odot) = 11.0$5 mag, SFR$\log(M_*/M_\odot) = 11.0$693 M$\log(M_*/M_\odot) = 11.0$7/yr
• Edge-on morphology with pronounced dust lane
• Transient offset by 0.67″ (5.5 kpc) in projection
• Among the most massive, luminous, and dusty long-GRB hosts

Spectra show strong molecular absorptions (CO, H$\log(M_*/M_\odot) = 11.0$8O) indicative of a significant evolved stellar population. No nuclear TDE or AGN activity is detected. The environment is interpreted as an extreme, perhaps merger-driven system (Gompertz et al., 26 Sep 2025, Sears et al., 16 Jun 2026).

4. Engine Models and Progenitor Hypotheses

Multiple engine models have been quantitatively developed to explain the phenomenology of GRB 250702B. The observational constraints collectively challenge all standard and non-standard progenitor frameworks:

A. Collapsar / Helium Merger

• Ultra-long accretion ($\log(M_*/M_\odot) = 11.0$9 s) and sub-second variability are difficult to reconcile with canonical collapsar models. However, a stellar-mass black hole merging with a helium star core within a common envelope can, in principle, extend accretion times to match $11.6$0, and also produce narrow jets and high $11.6$1 (Neights et al., 26 Sep 2025).
• Atypical collapsar models (fallback/accretion in supergiant envelopes) can produce a t$11.6$2 X-ray decay, highly stratified wind environments, and explain the long duration and intermittent prompt emission (Zhang et al., 30 Sep 2025).

B. Structured Precessing Magnetized Jet

• A rapidly spinning BH plus massive, misaligned debris disk launches a precessing “spine-sheath” magnetic jet. Lense–Thirring precession at $11.6$3--$11.6$4 yields the $11.6$5 s periodicity. Jet inclination and nutation modulate pulse visibility, explaining the observed “missing pulses” and opening-angle tension (An, 13 Nov 2025).

C. Tidal Disruption Scenarios

• Intermediate-mass black hole (IMBH) with repeated partial disruption of a white dwarf (WD) on a highly eccentric orbit can reproduce the prompt/intermittent structure, with orbital periods $11.6$61 hr, viscous flare durations $11.6$7100 s, and full disruption after 10–50 passages; relativistic precession causes only a fraction of jet-launch events to be visible (Sato et al., 1 Feb 2026, Yuan et al., 26 Feb 2026, Eyles-Ferris et al., 26 Sep 2025).
• Main-sequence star mTDE by $11.6$8 M$11.6$9 IMBH is consistent with afterglow energetics, stratified density, and timescales, but only for stars, not WDs, given fallback timing (Granot et al., 16 Dec 2025).

D. Micro–Tidal Disruption Event ($A_V$0TDE)

• Disruption of an ordinary star by a stellar-mass black hole or neutron star within a binary or after a natal kick can provide the required delay, duration, and energetic scaling. Both repeating (partial/free-fall) and full-dynamical disruptions are considered; fallback and accretion timescales naturally match the observed $A_V$1 day delay between precursor and main flare, and the multi-hour prompt (Beniamini et al., 26 Sep 2025).

E. Rejection of Ordinary TDE

• Classical TDEs by SMBHs (e.g., J1644+57 analogs) are not favored, as their timescales, energetics, and afterglow behavior diverge from those observed in GRB 250702B (Sears et al., 16 Jun 2026).

5. Temporal and Spectral Signatures

The event displays hallmark signatures distinguishing it from ordinary long GRBs and canonical TDEs:

• Quasi-periodic prompt emission episodes with $A_V$22825–4000 s separation; integer multiples to <0.03% (Levan et al., 18 Jul 2025)
• Candidate quasi-periodic oscillation at 0.046 Hz (21.7 s period) in later prompt emission, possibly linked to jet precession or magnetar-like activity (Song et al., 11 Oct 2025)
• High-energy spectral peaks at $A_V$3–$A_V$4 MeV with hard low-energy photon indices ($A_V$5), sub-second variability
• X-ray afterglow decay slope $A_V$6, matching $A_V$7 fallback accretion
• Deep late-time Chandra and JWST/NIRCam imaging show no bright SN, but obscuration and timing preclude ruling out faint or highly extinguished events
• Afterglow modeling supports both wind and Bondi-Hoyle type stratified media; radio afterglow consistent with an order-of-magnitude late-time enhancement predicted for repeated off-axis jet episodes in WD-IMBH partial disruption models (Sato et al., 1 Feb 2026)

6. Rate, Rarity, and Astrophysical Implications

GRB 250702B is among the rarest transients in the current high-energy sky; single-event detection in 16 years of Fermi/GBM all-sky coverage and strict beaming corrections imply intrinsic rates at least $A_V$8 times lower than long GRBs and $A_V$9 times below core-collapse supernovae (Gompertz et al., 26 Sep 2025). The host galaxy's extreme properties and the event’s location demand a strong role for galactic environment in shaping progenitor evolution and jet launching conditions.

A summary of plausible progenitor diagnostics is shown below:

Scenario	Duration ($2.8$0)	Delay/Recurrence	Host Offset	SN Constraint	Engine Duration	Spectral Hardness
Collapsar/Helium merger	Up to $2.8$1 s	None	Moderate	Faint/absent	Long	Yes
WD–IMBH Partial TDE	10$2.8$2–10$2.8$3 s	Quasi-periodic	Large	Absent	Repeating	Yes
Main-sequence mTDE (IMBH)	$2.8$4 s	None/mild	Large	Absent	Yes	Yes
Micro-TDE ($2.8$5TDE, stellar BH)	$2.8$6 s	Delay ($2.8$7day)	Off-nuclear	Absent	Yes	Yes
Structured, precessing jet	$2.8$8 s	$2.8$9	Any	—	Long	Hard, periodic

No single scenario is unequivocally supported, but WD–IMBH partial TDE, mTDE, and helium merger models remain competitive (Beniamini et al., 26 Sep 2025, Granot et al., 16 Dec 2025, Neights et al., 26 Sep 2025, Yuan et al., 26 Feb 2026).

7. Open Questions and Future Directions

Decisive discrimination among progenitor models will require:

• Deep late-time JWST/NIRCam imaging to template-subtract the host and isolate persistent or fading IR emission (Sears et al., 16 Jun 2026)
• Long-term X-ray (Chandra/XMM) monitoring to search for fallback-shutdown or TDE-like transitions, especially a $6.2$0 exponential break indicative of sub-Eddington accretion (O'Connor et al., 26 Sep 2025, Yuan et al., 26 Feb 2026)
• High-cadence radio calorimetry to test predictions of multi-jet models (off-axis jet brightenings) and constrain total energy budget (Sato et al., 1 Feb 2026)
• Searches for fainter or more extinguished associated SNe with high-sensitivity NIR spectroscopy (Gompertz et al., 26 Sep 2025)
• Statistical assessment of similar events in future all-sky transient surveys, potentially identifying new channels for ultra-long GRB production

The physical channel responsible for GRB 250702B remains uncertain, but the event has demonstrated the power of multi-wavelength, temporally dense observations and sophisticated modeling in testing the extremes of compact-object astrophysics (Gompertz et al., 26 Sep 2025, Sears et al., 16 Jun 2026, Zhang et al., 30 Sep 2025). The combination of energetics, host environment, and light-curve behavior positions GRB 250702B as a new benchmark for theories of relativistic transients.