GCN Circular 40398

SVOM/GRM team: Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)

SVOM/ECLAIRs team: Hui Yang, Laurent Bouchet (IRAP), Frédéric Piron (LUPM), Stéphane Schanne (CEA)

report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by a short burst GRB 250505A at 2025-05-05T02:10:34 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 narrow spike followed by a tail with a T90 of 0.26 +0.13/-0.07 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb250505A.png

The SVOM/GRM on-ground localization of this burst is (J2000):
RA: 55.6 deg
DEC: -39.0 deg
Error: 4.6 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 95 degrees from the SVOM optical axis. ECLAIRs was in operation during the burst but no significant signal is detected, which confirms that the burst occurred outside the ECLAIRs field of view.

With this localization, the time-averaged spectrum from T0+0.02 to T0+0.15 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.10 +0.37/-0.35 and the cutoff energy, parameterized as Epeak, is 70 +/-10 keV. The event fluence (10-1000 keV) in this time interval is (5.9 +0.6/-0.5)E-07 erg/cm^2.

The localization of GRB 250505A in the 'Amati' relation diagram is shown at: 
https://www.bursthub.cn//admin/static/grb250505A_amati.png

We note that the Epeak of this burst is relatively soft in short GRBs, while the hardness and duration are both similar to 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)