On the Ultra-Long Gamma-Ray Transient GRB 250702B/EP250702

TL;DR

GRB 250702B/EP250702a challenges simple GRB classification by exhibiting an ultra-long gamma-ray phase with a detectable precursor ~25 hours before a four-episode main burst, followed by a long soft X-ray tail. The authors perform a wide temporal, multi-instrument analysis using ETJASMIN across Insight-HXMT, GECAM, and Fermi/GBM, complemented by X-ray follow-up, to construct a complete high-energy history spanning about 40 days. The gamma-ray and X-ray phenomenology—precursor, episodic main burst, and a $t^{-5/3}$ decay in soft X-rays—fits a ULGRB collapsar powered by fallback accretion in a supergiant progenitor, rather than a jetted TDE. This work demonstrates the value of extended, multi-instrument monitoring for extreme transients and identifies GRB 250702B/EP250702a as the longest ULGRB observed to date, with implications for jet physics and progenitor demographics.

Abstract

GRB 250702B/EP250702a is an interesting long-duration gamma-ray transient whose nature is in debate. To obtain a full picture in gamma-ray band, we implement a comprehensive targeted search of burst emission in a wide window of 30 days jointly with Insight-HXMT, GECAM and Fermi/GBM data within the ETJASMIN framework. In gamma-ray band, we find there is a 50-second precursor about 25 hours before the 4-hour main burst, which generally consists of 4 emission episodes. Remarkably, we find that the soft X-ray emission (after the main burst) decays as a power-law with start time aligning with the last episode of main emission and index of -5/3 perfectly consistent with the canonical prediction of fallback accretion. We conclude that the properties of precursor, main burst and the following soft X-ray emission strongly support the atypical collapsar Ultra-Long Gamma-Ray Burst (ULGRB) scenario rather than the Tidal Disruption Event (TDE), and all these gamma-ray and soft X-ray emission probably originate from relativistic jet whose luminosity is dominated by the fallback accretion rate during the death collapse of a supergiant star.

Figures (6)

• Figure 1: Panel (a): Monitoring periods for each instrument: Insight-HXMT/HE, GECAM-B or Fermi/GBM. Each column represents one day. Time goes by from up to down in each column. Panel (b): Coverage percentage for each instrument or joint observations. The joint monitored time intervals cover 81.4% of the 30-day search. Panel (c): Monitored time intervals from precursor to end of main burst stage for each instrument or joint observations.
• Figure 2: Panel (a): Gamma-ray and soft X-ray light curves of GRB 250702B/EP250702a in a wide time window of about 40 days. Gamma-ray light curves are background subtracted and normalized by receiving area of detectors, from Insight-HXMT/HE, GECAM-B and Fermi/GBM. Precursor light curves of Insight-HXMT/HE and Fermi/GBM, and main burst light curve of GECAM-B are binned in 30 seconds, while others in 60 seconds. Soft X-ray data are well fitted with a power-law with index of around -5/3. Details of data analyses could be found in text and appendix. Panel (b) - (f): Light curves of precursor (P) and four episodes (M1 to M4) of main burst, binned in 1 second.
• Figure 3: The spectral fit result of the precursor candidate with power-law model.
• Figure 4: Soft X-ray emission of GRB 250702B/EP 250702a in comparison with GRBs including some ULGRBs. Redshift of GRB 250702B derived from gompertz_jwst_2025
• Figure 5: Relation between precursor waiting time and main burst duration.