TITLE: GCN CIRCULAR NUMBER: 41229 SUBJECT: GRB 250804B: SVOM/GRM observation of a short burst DATE: 25/08/04 16:01:26 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: Jean-Luc Atteia, Sebastien Guillot (IRAP)
Report on behalf of the SVOM team:
SVOM/GRM was triggered in-flight by GRB 250804B (SVOM trigger reference: sb25080401) at 2025-08-04T04:44:51.600 UTC (T0).
With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of a single pulse with a T90 of 0.37 +0.09/-0.15 s in the 15-5000 keV band.
The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb250804B.png
The SVOM/GRM on-ground localization of this burst is (J2000): RA: 36.3 deg DEC: -42.1 deg Error: 4.3 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 56 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. Nonetheless, the burst is seen (via analysis of X-band data) in the ECLAIRs data above 50 keV at the T0 given by GRM, likely through the ECLAIRs shielding.
With this localization, the time-averaged spectrum from T0-0.5 to T0+0.4 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.78 +0.07/-0.08 and the cutoff energy, parameterized as Epeak, is 1650 +38/-28 keV. The event fluence (10-1000 keV) in this time interval is (2.63 +/-0.13)E-06 erg/cm^2. Thus GRB 250804B is consistent with Type II GRBs in the 'Amati' relation diagram, as shown at: https://www.bursthub.cn//admin/static/grb250804B_amati.png
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)
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