TITLE: GCN CIRCULAR NUMBER: 40120 SUBJECT: GRB 250407A: SVOM/GRM observation of a bright long burst DATE: 25/04/08 14:40:37 GMT FROM: Chenwei Wang at IHEP cwwang@ihep.ac.cn
SVOM/GRM team: Chen-Wei Wang, Yue Huang, Shi-Jie Zheng, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)
SVOM/ECLAIRs team: Marius Brunet, Jean-Luc Atteia, Hui Yang (IRAP), Maria-Grazia Bernardini (INAF-OAB), Stéphane Schanne (CEA),
Report on behalf of the SVOM team:
SVOM/GRM was triggered in-flight by GRB 250407A (SVOM trigger reference: sb25040701) at 2025-04-07T15:48:13.000 (T0). This burst was also detected by Fermi/GBM (Fermi GBM Team, GCN 40104).
With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of multi-pulses with a T90 of 11.1 +0.2/-0.3 s in the 15-5000 keV band.
The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb250407A.png
In addition, ECLAIRs onboard trigger also detected a very significant increase of the count-rates at the time of the burst, but the detector plane image is not correlated with the mask pattern. This confirmed that the position of this burst, which is determined by Swift/XRT (GCN 40113, RA: 110.75876 deg, DEC: 36.79827 deg, Error: 2.8 arcsec), located at about 45.6 degrees from the SVOM optical axis, and was just outside the ECLAIRs field of view.
With this localization, the time-averaged spectrum from T0 to T0+18 s is best fitted by Band function. The alpha is -0.90 +/- 0.01, beta is -2.27 +/- 0.09 and the cutoff energy, parameterized as Epeak, is 569 +/- 27 keV. The event fluence (10-1000 keV) in this time interval is (1.08 +/- 0.01)E-04 erg/cm^2. Thus GRB 250407A is consistent with Type II GRBs in the 'Amati' relation diagram, as shown at: https://www.bursthub.cn//admin/static/grb250407A_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/GRM point of contact for this burst is: Chen-Wei Wang (IHEP) (cwwang@ihep.ac.cn)
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