On the Ultra-Long Gamma-Ray Transient GRB 250702B/EP250702
Authors
Jin-Peng Zhang
Chen-Wei Wang
Zheng-Hang Yu
Shao-Lin Xiong
Shu-Xu Yi
Jia-Cong Liu
Wang-Chen Xue
Wen-Jun Tan
Zi-Rui Zhang
Rahim Moradi
Hao-Xuan Guo
Chao Zheng
Yan-Qiu Zhang
Yue Wang
Sheng-Lun Xie
Peng Zhang
Yang-Zhao Ren
Cheng-Kui Li
Xiao-Bo Li
Ce Cai
Shuo Xiao
Li-Ming Song
Shuang-Nan Zhang
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.
