TITLE: GCN CIRCULAR NUMBER: 41383 SUBJECT: GRB 250813B: SVOM/GRM observation DATE: 25/08/15 07:23:53 GMT FROM: tanwj@ihep.ac.cn
SVOM/GRM team: Wen-Jun Tan, Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)
SVOM/ECLAIRs team: Stéphane Schanne
Report on behalf of the SVOM team:
SVOM/GRM was triggered in-flight by GRB 250813B (SVOM burst-id sb25081303) at 2025-08-13T22:51:12UTC (T0), which is also detected by SVOM/ECLAIRs (W.J. XIE et al., GCN #41354), Swift-XRT (K.L. Page et al., GCN #41359), EP-FXT (Q. Y. Wu et al., GCN #41361), SVOM/MXT (D. Götz et al., GCN #41368) and FERMI/GBM (Jacob Smith et al., GCN #41376)
With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of multiple pulses with a T90 of 24.9 +/-0.76 s in the 15-5000 keV band.
The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb250813B.png
With this localization given by SVOM/ECLAIRs (RA= 336.728, DEC= 12.498, GCN #41354), the time-averaged spectrum from T0-6 to T0+35 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.84 +/-0.07 and the cutoff energy, parameterized as Epeak, is 109 +6/-5 keV. The event fluence (10-1000 keV) in this time interval is (1.35 +/-0.20)E-05 erg/cm^2.
With the redshift z = 1.752 by VLT/X-shooter (B. Schneider et al., GCN#41363), the Eiso (from 1 keV to 10 MeV in rest frame) of this burst is (1.16 +0.20/-0.18)E53 erg. GRB 250813B is consistent with Type II GRBs in the 'Amati' relation diagram, as shown at: https://www.bursthub.cn//admin/static/grb250813B_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: Wen-Jun Tan (IHEP)(tanwj@ihep.ac.cn)
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