TITLE: GCN CIRCULAR NUMBER: 29889 SUBJECT: GRB 210421A: GECAM detection DATE: 21/04/23 06:37:40 GMT FROM: QiBin Yi at IHEP, HXMT S. L. Xie, X. Y. Song, Q. B. Yi, X. Y. Zhao, Y. Huang, S. L. Xiong, S. Xiao, C. Cai, Y. Zhao, J. C. Liu, Y. Q. Zhang, W. C. Xue, C. Zheng, Z. W. Guo, C. W. Wang, D. Y. Guo, P. Wang, X. B. Li, W. X. Peng, R. Qiao, Z. H. An, C. Chen, G. Chen, W. Chen, M. Gao, K. Gong, D. Y. Guo, J. J. He, B. Li, C. Li, C. Y. Li, J. H. Li, Q. X. Li, X. Q. Li, Y. G. Li, X. H. Liang, J. Y. Liao, J. C. Liu, X. J. Liu, Y. Q. Liu, F. J. Lu, Q. Luo, X. Ma, G. Ou, D. L. Shi, J. Y. Shi, L. M. Song, G. X. Sun, X. L. Sun, Y. L. Tuo, J. Z. Wang, X. Y. Wen, Y. B. Xu, Y. P. Xu, S. Yang, M. Yao, B. X. Zhang, C. Y. Zhang, D. L. Zhang, Fan Zhang, Fei Zhang, H. M. Zhang, K. Zhang, P. Zhang, S. N. Zhang, Z. Zhang, S. Y. Zhao, S. J. Zheng, X. Zhou (IHEP), report on behalf of GECAM team: During the commissioning phase, GECAM-B triggered a long burst, GRB 210421A, at 2021-04-21T00:27:32.000 UTC (denoted as T0), which was also detected by Swift/BAT(GCN #29849). According to the GECAM-B light curves in about 8 keV - 115 keV, this burst mainly consists of multiple pulses with duration of about 20 s. Although in-flight calibration has not been finalized yet, GECAM-B localized this burst to the following position (J2000): Ra: 103.56 deg Dec: 2.26 deg Err: 3.46 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. GECAM location is consistent with the Swift/BAT(GCN #29849) position within the error. The GECAM light curve could be found here: http://twiki.ihep.ac.cn/pub/GECAM/GRBList/multiLCvsE_2021-04-21T003A273A32.950_.pdf 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).