TITLE: GCN CIRCULAR NUMBER: 30798 SUBJECT: GRB 210909B: GECAM detection DATE: 21/09/10 15:02:21 GMT FROM: Shuo Xiao at IHEP <xiaoshuo@ihep.ac.cn> S. Xiao, S. L. Xiong, Y. Zhao, X. Y. Zhao, Y. Huang, X. Y. Song, C. Cai, S. L. Xie, Y. Q. Zhang, Z. W. Guo, J. C. Liu, C. Zheng, W. C. Xue, 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, X. L. Zhang, 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 burst, GRB 210909B, at 2021-09-09T20:02:33.600 UTC (denoted as T0), which was also detected by INTEGRAL/SPI-ACS. According to the GECAM-B light curves in about 15-3500 keV, this burst mainly consists of a broad pulse with a duration of about 10 s. The GECAM light curve could be found here: http://twiki.ihep.ac.cn/pub/GECAM/GRBList/utn210909_200233_GECAMb.png GECAM-B localized this burst to the following position (J2000): Ra: 358.6 deg Dec: -74.9 deg Err: 1.1 deg (1-sigma, statistical only) The systematic error of location is estimated to be several degrees. 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).