TITLE: GCN CIRCULAR NUMBER: 43122 SUBJECT: GRB 251209A: GECAM-B observation of a long burst DATE: 25/12/15 13:57:46 GMT FROM: guohx@ihep.ac.cn Hao-Xuan Guo, Chen-Wei Wang, Sheng-Lun Xie, Yang-Zhao Ren, Peng Zhang, Yue Wang, Shao-Lin Xiong (IHEP) report on behalf of GECAM team: GECAM-B was triggered in-flight by a long burst GRB 251209A at 2025-12-09T13:24:22.000 UTC (denoted as T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #43048 and GCN #43068), Global MASTER-Net (V.Lipunov et al., GCN#43053), AstroSat CZTI (M. Tembhurnikar et al., GCN#42996), NuSTAR (G. Waratkar et al., GCN #43078) and Insight-HXMT (Hao-Xuan Guo et al., GCN #43119). According to the GECAM-B light curves in about 70-6000 keV, this burst mainly consists of multiple pulses main emission with a duration (T90) of 40.5 +/-1.7 s. The GECAM-B light curve can be found here: https://www.bursthub.cn//admin/static/gecamgrb251209A.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). View this GCN Circular online at https://gcn.nasa.gov/circulars/43122. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...