TITLE: GCN CIRCULAR NUMBER: 40396 SUBJECT: GRB 250506A: GECAM detection DATE: 25/05/08 14:13:56 GMT FROM: Chenwei Wang at IHEP cwwang@ihep.ac.cn

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

GECAM-B was triggered in-flight by GRB 250506A at 2025-05-06T02:23:22.850 UTC (denoted as T0), which was also detected by Fermi/GBM (Fermi GBM team, GCN #40355) and SVOM (Wang et al., GCN #40358).

According to the GECAM-B light curves in about 40-6000 keV, this burst mainly consists of multiple pulses with a duration (T90) of 34.6 +0.8/-1.9 s.

The GECAM-B light curve can be found here: https://www.bursthub.cn//admin/static/gecamgrb250506A.png

With the localization of RA, Dec = 219.28269, +28.87835, as determined by Swift/XRT (D.N. Burrows et al., GCN #40394), the time-averaged spectrum from T0-10s to T0+60 s is best fitted by a powerlaw with an alpha of -2.02 +/- 0.11. The event fluence (10-1000 keV) in this time interval is (2.60 +/- 0.14)E-05 erg/cm^2.

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).

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