On the Ultralong Gamma-Ray Transient GRB 250702B/EP 250702a

Abstract GRB 250702B/EP 250702a is an interesting long-duration gamma-ray transient whose nature is in debate. To obtain a full picture in the 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 Gamma-ray Burst Monitor data within the ETJASMIN framework. In the gamma-ray band, we find that there is a 50 s precursor about 25 hr before the 4 hr main burst, which generally consists of four emission episodes. Remarkably, we find that the soft X-ray emission after the main burst decays as a power law with a start time aligning with the last episode of main emission and an index of −5/3, which is perfectly consistent with the canonical prediction of fallback accretion. We conclude that the properties of the precursor, the main burst, and the following soft X-ray emission strongly support the ultralong gamma-ray burst (ULGRB) scenario, and all this gamma-ray and soft X-ray emission probably originates from a relativistic jet whose luminosity is dominated by the fallback accretion rate during the death collapse of a supergiant star.