TITLE: GCN CIRCULAR NUMBER: 43119 SUBJECT: GRB 251209A: Insight-HXMT/HE detection DATE: 25/12/15 13:24:01 GMT FROM: guohx@ihep.ac.cn Hao-Xuan Guo, Chen-Wei Wang, Cheng-Kui Li, Shao-Lin Xiong, and Chao Zheng (IHEP) report on behalf of the Insight-HXMT team: At 2025-12-09T13:24:31.000 (T0), Insight-HXMT/HE detected a long burst, GRB 251209A, 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) and NuSTAR (G. Waratkar et al., GCN #43078). The Insight-HXMT/HE light curve mainly consists of multiple pulses with T90 of 43.0 +13.5/-10.0 s. The 1s peak rate, measured from T0+0.25 s, is 822 cnts/sec. Insight-HXMT/HE detected a total of 12,343 counts from this burst. The Insight-HXMT /HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb251209A.png All measurements above are made with the CsI detectors of Insight-HXMT/HE operating in the Low-Gain mode with the energy range of about 300-3000 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope. Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://www.hxmt.org View this GCN Circular online at https://gcn.nasa.gov/circulars/43119. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...