TITLE: GCN CIRCULAR NUMBER: 40184 SUBJECT: GRB 250416C / EP250416a: SVOM/GRM observation of a burst DATE: 25/04/20 12:06:30 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: Maria-Grazia Bernardini (INAF-OAB), Stéphane Schanne (CEA), Frédéric Piron (LUPM)

Report on behalf of the SVOM team:

SVOM/GRM was triggered on-ground by GRB 250416C at 2025-04-16T17:53:48 (T0). This burst was also detected by Konus-Wind (D. Svinkin et al, GCN 40167) and EP (Zhao et al., GCN 40154; Zhou et al., GCN 40165).

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 8 +4/-4 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb250416C.png

The time coincidence and relative counts of each detectors of GRM suggest that this GRB is associated with EP250416a. In addition, the position of this burst, which is determined by EP/FXT (GCN 40154, RA: 256.4228 deg, DEC: 25.7755 deg, Error: 20 arcsec), located at about 83 degrees from the SVOM optical axis, and was outside the ECLAIRs field of view.

With this localization, the time-averaged spectrum from T0-10 to T0+10 s is best fitted by a power law function. The power law index is -2.04 +0.19/-0.26. The event fluence (10-1000 keV) in this time interval is (1.2 +0.3/-0.3)E-06 erg/cm^2.

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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