TITLE: GCN CIRCULAR NUMBER: 40808 SUBJECT: GRB 250621B: SVOM/GRM detection of a possible magnetar X-ray burst DATE: 25/06/22 06:07:38 GMT FROM: Chenwei Wang at IHEP <cwwang@ihep.ac.cn> SVOM/GRM team: Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP) SVOM/ECLAIRs team: Frédéric Piron (LUPM) Report on behalf of the SVOM team: SVOM/GRM was triggered in-flight by a short burst GRB 250621B (SVOM trigger reference: sb25062104) at 2025-06-21T09:13:47.100 UTC (T0), which is also detected by Fermi/GBM (TrigNum 772190032). With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of a narrow spike with a T90 of 0.11 +0.02/-0.03 s in the 15-5000 keV band. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb250621B.png The SVOM/GRM on-ground localization of this burst is (J2000): RA: 273.9 deg DEC: -24.8 deg Error: 2.9 deg (1sigma, statistical only) We caution that the calibration of SVOM/GRM is undergoing and this localization is subject to systematic errors. In addition, the position of this burst, as determined by GRM, is located at about 8 degrees from the SVOM optical axis, but ECLAIRs was not collecting data at the time of this burst. With this localization, the time-averaged spectrum from T0-0.3 to T0+0.2 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.05 +0.25/-0.27 and the cutoff energy, parameterized as Epeak, is 31 +/- 3 keV. The event fluence (10-1000 keV) in this time interval is (3.79 +/-0.15)E-07 erg/cm^2. We note that the localization of this burst is relatively close to the Galactic disk and compatible with the position of SGR 1806-20, as well as the hardness and duration all suggest a magnetar X-ray burst. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS. The SVOM point of contact for this burst is: Chen-Wei Wang (IHEP)(cwwang@ihep.ac.cn) View this GCN Circular online at https://gcn.nasa.gov/circulars/40808. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...