GRB 250702BDE: Ultra-long Gamma-Ray Burst

• GRB 250702BDE is an ultra-long gamma-ray burst characterized by a prolonged, multi-episode prompt emission with a unique day-long precursor.
• It exhibits a complex spectral evolution and afterglow behavior across X-ray, gamma-ray, and infrared wavelengths in a heavily dust-obscured, off-nuclear host galaxy.
• Comprehensive modeling supports exotic progenitor scenarios, including intermediate-mass black hole tidal disruptions and helium merger events, challenging conventional collapsar models.

GRB 250702BDE denotes an extraordinary gamma-ray burst (GRB) detected in July 2025, exhibiting the longest duration and most complex temporal and spectral structure among all confirmed GRBs. This event includes both soft X-ray and gamma-ray emission over a time span exceeding 25,000 s, with multiple distinct gamma-ray episodes and a faint, gradual X-ray precursor detected nearly a day before the main activity. The counterpart is localized to a massive, dusty, highly extended host at $z=1.036$, distinctly offset from the host nucleus, and the afterglow is characterized by extreme reddening and standard fireball afterglow properties. GRB 250702BDE strongly challenges standard collapsar, compact binary merger, and canonical tidal disruption event (TDE) paradigms, stimulating new progenitor models involving ultra-long fallback collapsars, common-envelope helium mergers, and intermediate-mass black hole (IMBH) tidal disruptions.

1. Temporal and Spectral Phenomenology

GRB 250702BDE's multi-episode prompt emission, extreme duration, and structured X-ray precursor distinguish it from all previously cataloged GRBs or TDEs. The timeline is as follows (Levan et al., 18 Jul 2025, Zhang et al., 30 Sep 2025, Neights et al., 26 Sep 2025, Granot et al., 16 Dec 2025):

• Precursor (X-ray/soft gamma-ray phase):
  • Detected by the Einstein Probe $\sim$24–25 h prior to the first main Fermi/GBM gamma-ray trigger.
  • Duration $\sim$50 s, spectrum fit by a pure power law ($\Gamma_P=1.65\pm0.10$).
  • Isotropic equivalent energy $E_{\text{iso,P}}\sim10^{52}$ erg (for $z=1.036$).
• Main gamma-ray burst:
  • Spans $T_{90}\simeq2.5\times10^4$ s, consisting of three to four gamma-ray episodes (with "D", "B", "E" designations in Fermi-GBM, as well as substructure).
  • Individual bright episodes present quasi-periodic separation ($\Delta t\sim$4000 s), suggesting underlying engine modulation.
  • Integrated isotropic energy $E_{\gamma,\text{iso}}\gtrsim2.2\times10^{54}$ erg.
  • Spectra: Band function fits with $E_\mathrm{peak}=500\to200$ keV, $\sim$0, $\sim$1. Some intervals show $\sim$2 MeV and $\sim$3.
  • Hardness decreases over time: M1 is the hardest, with each subsequent episode softer.
  • Subsecond minimum variability timescale of $\sim$4 s.
• Afterglow and soft X-ray emission:
  • Swift/XRT and Chandra monitor the afterglow from 0.5 days to 65 days.
  • X-ray flux decays as $\sim$5 with $\sim$6–1.9; late-time IR photometry suggests a power-law decline with possible late flattening.

The following table provides a compact summary of episode properties (Levan et al., 18 Jul 2025, Zhang et al., 30 Sep 2025, Granot et al., 16 Dec 2025, Neights et al., 26 Sep 2025):

Phase	Start Time (UTC)	Duration / Spacing	Energy / Flux
X-ray Precursor	2025-07-01 11:55:17	$\sim$750 s	$\sim$8 erg
Main GRB	2025-07-02 $\sim$913h	$\sim$04-7 h (total)	$\sim$1 erg
Sub-episodes	$\sim$213:09–16:21	$\sim$3 s	Peaks $\sim$4–$\sim$5 MeV
Afterglow	$\sim$60.5–65 days	$\sim$7	$\sim$8 erg

2. Multi-Wavelength Counterpart and Host Galaxy

Extensive follow-up in the radio, near-IR, and optical bands established the extragalactic, off-nuclear nature of GRB 250702BDE and provided stringent constraints on both the afterglow and host galaxy (Levan et al., 18 Jul 2025, Sears et al., 16 Jun 2026, Gompertz et al., 26 Sep 2025):

• Counterpart localization: Precise coordinates with $\sim$90.7″ ($\Gamma_P=1.65\pm0.10$05.7 kpc) offset from the host's center, far from the stellar nucleus.
• Afterglow properties: Extremely red spectral energy distribution (SED) with $\Gamma_P=1.65\pm0.10$1–11 mag host extinction. Fades as a standard synchrotron power-law with spectral and temporal indices consistent with forward-shock and reverse-shock models in a stratified circumburst medium $\Gamma_P=1.65\pm0.10$2, $\Gamma_P=1.65\pm0.10$3–2.0.
• Host galaxy:
  • Massive ($\Gamma_P=1.65\pm0.10$4–11.6), dusty ($\Gamma_P=1.65\pm0.10$5–6.2 mag), large (half-light radius $\Gamma_P=1.65\pm0.10$6 kpc), with elevated star-formation rate (SFR$\Gamma_P=1.65\pm0.10$793 $\Gamma_P=1.65\pm0.10$8 yr$\Gamma_P=1.65\pm0.10$9).
  • Edge-on disk, strong dust lane, and location within the $E_{\text{iso,P}}\sim10^{52}$00.1% most massive and dust-obscured star-forming galaxies at $E_{\text{iso,P}}\sim10^{52}$1.
• Supernova constraints: JWST and NIRSpec spectroscopy finds no evidence for a luminous supernova coincident with the afterglow, limiting associated SN Ic-BL emission to $E_{\text{iso,P}}\sim10^{52}$2–0.5 $E_{\text{iso,P}}\sim10^{52}$3 SN 2023lcr luminosity for lines of sight of moderate ($E_{\text{iso,P}}\sim10^{52}$4 mag) extinction (Gompertz et al., 26 Sep 2025).

3. Physical Modeling and Afterglow Fits

The broadband afterglow spanning radio through X-ray is modeled successfully with standard relativistic blastwave (fireball) theory plus high host extinction. MCMC fits yield (Levan et al., 18 Jul 2025, Granot et al., 16 Dec 2025, O'Connor et al., 26 Sep 2025):

• Energetics:
  • $E_{\text{iso,P}}\sim10^{52}$5–8$E_{\text{iso,P}}\sim10^{52}$6 erg (forward shock).
  • Jet opening angle $E_{\text{iso,P}}\sim10^{52}$7–$E_{\text{iso,P}}\sim10^{52}$8 (narrow, consistent with strong collimation).
  • Prompt efficiency $E_{\text{iso,P}}\sim10^{52}$9%.
• Environment:
  • Best-fit density slope $z=1.036$0, compatible with Bondi accretion of the ISM by a compact object or a wind medium from a massive progenitor.
  • Pivot density $z=1.036$1 cm$z=1.036$2 at $z=1.036$3 cm.
• Temporal evolution:
  • Radio SEDs show both reverse and forward-shock contributions.
  • X-ray afterglow decay and late IR data broadly follow $z=1.036$4, with tentative evidence (JWST/NIRCam) for late-time flattening possibly consistent with either plateau afterglow or TDE scenarios (Sears et al., 16 Jun 2026).

4. Progenitor Scenarios and Theoretical Interpretations

No single progenitor class fully accounts for the duration, structured repetition, extreme energetics, and environment of GRB 250702BDE. The main models considered are (Levan et al., 18 Jul 2025, Zhang et al., 30 Sep 2025, Neights et al., 26 Sep 2025, Granot et al., 16 Dec 2025, Sears et al., 16 Jun 2026):

A) Ultra-Long Collapsar (Massive Star Collapse)

• Fallback accretion from an extended blue or red supergiant can generate durations of several hours.
• Predicts X-ray flux evolution $z=1.036$5 from fallback, jet power $z=1.036$6 matching the observed decay (Zhang et al., 30 Sep 2025).
• However, the presence of a long ($z=1.036$71 d) precursor is not naturally accommodated, and a standard collapsar cannot produce day-length engine activity without invoking exotic fallback or disk instabilities (O'Connor et al., 26 Sep 2025, Neights et al., 26 Sep 2025).

B) Helium Merger (BH+He Star, Common Envelope)

• Inspiral of a BH into a stripped He star forms a highly rotationally supported, massive accretion disk with $z=1.036$8–$z=1.036$9 s.
• Efficient jet launch via the Blandford-Znajek mechanism can generate both the high $T_{90}\simeq2.5\times10^4$0 and narrow collimation; naturally extended durations; subsecond variability.
• Explains many properties but not the day-long precursor or off-nuclear location without additional assumptions (Neights et al., 26 Sep 2025).

C) Tidal Disruption Event (TDE) by IMBH

• GRB 250702BDE's offset, long duration, highly stratified environment, and precursor can be explained by disruption of a main-sequence star by an IMBH ($T_{90}\simeq2.5\times10^4$1–$T_{90}\simeq2.5\times10^4$2).
• Afterglow modeling based on Bondi/IMBH parameters gives $T_{90}\simeq2.5\times10^4$3 (with $T_{90}\simeq2.5\times10^4$4 km s$T_{90}\simeq2.5\times10^4$5), consistent with all afterglow and variability constraints.
• Timescales (fallback $T_{90}\simeq2.5\times10^4$6 s, circularization/viscous $T_{90}\simeq2.5\times10^4$7few$T_{90}\simeq2.5\times10^4$8 s) match pre-peak and main emission exactly for an MS star, but not a WD TDE (which would require $T_{90}\simeq2.5\times10^4$9 tens of seconds).
• The multi-episodic prompt activity and quasi-periodic structure are compatible with disk instabilities or partial mass fallback; the X-ray/IR light curve rise is explainable by disk buildup (Granot et al., 16 Dec 2025).

D) White Dwarf Tidally Disrupted by IMBH

• Repeating, integer-timed flares are a natural outcome for partial WD–IMBH TDEs with eccentric orbits (Levan et al., 18 Jul 2025).
• However, the observed prompt and precursor timescales (%%%%70$\sim$071%%%% s) exceed all WD TDE predictions, and the total energetics challenge such models unless extreme beaming or efficiency is assumed (Granot et al., 16 Dec 2025).

E) Hybrid or Micro-TDE

• Disruption of a star by a stellar-mass BH (micro-TDE) or a binary He-star + BH merger with extended fallback phase can, under some circumstances, match the observed duration, variability, and non-nuclear location (O'Connor et al., 26 Sep 2025).
• The main observational discriminants are late-time X-ray shutdown (indicative of Eddington-limited fallback), presence (or lack) of a supernova, and continued afterglow evolution in radio/X-ray.

5. Discriminating Observational Signatures and Event Rates

GRB 250702BDE's rarity, extreme energetics, and unique environment constrain progenitor channels and cosmic event rates (Gompertz et al., 26 Sep 2025, O'Connor et al., 26 Sep 2025):

• Beaming-corrected rate: $\Delta t\sim$2 yr$\Delta t\sim$3 Gpc$\Delta t\sim$4, $\Delta t\sim$51000$\Delta t\sim$6 lower than ordinary GRBs.
• Jet opening ($\Delta t\sim$7) and energetics: Consistently inferred to be $\Delta t\sim$8, resulting in strong on-axis selection.
• Host properties: Its massive, dust-rich, spatially extended host is exceptionally rare among GRB hosts, suggesting conditions conducive to either IMBH seeding, He-star mergers, or rare massive star populations.
• Future distinguishing diagnostics:
  • Abrupt jet-shutdown in the X-ray afterglow would support a TDE origin.
  • Late-time IR plateau or flattening, in excess of extrapolated forward shock, as seen in jetted TDEs.
  • Detection or exclusion of a supernova bump coincident with the afterglow (as in classical collapsars).
  • Continued VLBI monitoring for radio expansion and size constraints on deceleration physics (Sears et al., 16 Jun 2026, Granot et al., 16 Dec 2025).

6. Outstanding Questions and Prospects

Despite an extensive multi-instrument campaign, decisive discrimination among collapsar, helium merger, and IMBH mTDE origins for GRB 250702BDE remains open due to model degeneracies and host extinction limitations. The following key points remain (Levan et al., 18 Jul 2025, Zhang et al., 30 Sep 2025, Sears et al., 16 Jun 2026, O'Connor et al., 26 Sep 2025, Granot et al., 16 Dec 2025):

• The phenomenologically favored model is a TDE of a main-sequence star by an off-nuclear IMBH, given the timescales, rise/decay profiles, variability, afterglow environment, and localization.
• Helium merger models remain plausible, especially in the presence of recent star formation or binary channels in massive hosts.
• Ultra-long fallback collapsars, while matching some characteristics (power-law decay, high $\Delta t\sim$9), are disfavored by the long precursor and event morphology.
• White dwarf TDEs are ruled out by duration and energetics for main prompt phases.

Future observations with JWST/NIRCam, late-time X-ray monitoring (Chandra, Athena), radio VLBI, and deep host-galaxy IFU spectroscopy are expected to constrain progenitor properties, clarify the contribution of delayed supernova components, and potentially confirm the first clear extragalactic IMBH mTDE with high-energy outflows. GRB 250702BDE thus establishes a new benchmark for probing the physics of ultra-long relativistic transients and constraining rare black hole populations in the Universe.