GCN Circular 44473

Chen-Wei Wang, Shao-Lin Xiong, Chao Zheng (IHEP) report on behalf of GECAM team:

GECAM-B was triggered in-flight by a hard burst GRB 260504B at 2026-05-04T09:31:19.450 UTC (denoted as T0). According to the GECAM-B light curves in about 70-6000 keV, this burst mainly consists of a single pulse with a duration (T90) of 0.20 +0.02/-0.06 s, which is also detected by SVOM/ECLAIRs and SVOM/GRM (A. Saccardi et al., GCN # 44454).

The GECAM-B light curve can be found here:
https://www.bursthub.cn//admin/static/gecambgrb260504B.png

The time-averaged spectrum from T0-0.07 s to T0+0.1 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.07 +0.29/-0.27 and the cutoff energy, parameterized as Epeak, is 1240 +730/-400 keV. The event fluence (10-1000 keV) in this time interval is (1.06 +0.13/-0.10)E-06 erg/cm^2. Thus GRB 260504B is consistent with Type I GRBs in the 'Amati' relation diagram, as shown at: 
https://www.bursthub.cn//admin/static/gecambgrb260504B_amati.png

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).