TITLE: GCN CIRCULAR NUMBER: 30881 SUBJECT: GRB 210925A: GECAM detection DATE: 21/09/26 09:27:52 GMT FROM: Chao Zheng at IHEP <zhengchao97@ihep.ac.cn> C. Zheng, S. L. Xiong, S. Xiao, Y. Q. Zhang, W. C. Xue, X. Y. Song, X. Y. Zhao, Y. Huang, S. L. Xie, C. Cai, Y. Zhao, Z. W. Guo, X. L. Zhang, C. Y. Li, C. W. Wang, Q. B. Yi, B. X. Zhang, P. Zhang, W. X. Peng, R. Qiao, D. Y. Guo, X. B. Li, X. Ma, P. Wang, J. Wang, Z. Zhang, S. J. Zheng, W. Chen, J. J. He, G. Y. Zhao, Y. Q. Du, H. Wu, J. Liang, Q. Luo, J. C. Liu, L. M. Song, F. J. Lu, S. N. Zhang (IHEP), report on behalf of GECAM team: During the commissioning phase, GECAM-B was triggered in-flight by a long bright burst, GRB 210925A, at 2021-09-25T19:12:34.600 UTC (denoted as T0), which was also detected by Fermi/GBM (GCN #30880). Its alert data was promptly downlinked to the ground through the short message service of BeiDou Navigation Satellite System (BDS). The time latency of the first BeiDou message relative to the trigger time is about 1 minute. According to the BDS alert data, this burst mainly consists of multiple pulses with duration of about 10 s. The GECAM light curve could be found here: http://twiki.ihep.ac.cn/pub/GECAM/GRBList/utn210925_191234_GECAMb.png An automatic in-flight localization was calculated using the light curves and spectrum. Although the in-flight calibration of energy response and l ocalization has not been finalized yet, GECAM-B localized this burst to the following position (J2000): Ra: 357.83 deg Dec: -24.58 deg Err: 3.50 deg (1-sigma, statistical only) The current systematic error of location is estimated to be several degrees which could be minimized by the ongoing calibration. Please note that all GECAM results here are preliminary. The final analysis will be published in journal papers or GECAM online catalog. Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission consists of two small satellites (GECAM-A and GECAM-B) in Low Earth Orbit (600 km, 29 deg), launched on Dec 10, 2020 (Beijing Time), which was funded by the Chinese Academy of Sciences (CAS).