TITLE: GCN CIRCULAR NUMBER: 39164 SUBJECT: GRB 250127C: SVOM/GRM observation of a burst DATE: 25/02/06 07:00:22 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: Jean-Luc Atteia (IRAP), Stéphane Schanne (CEA), Frédéric Piron (LUPM)

report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by a long burst GRB 250127C (SVOM trigger reference: sb25012702) at 2025-01-27T05:34:47.500 UTC (T0).

With the event-by-event data downloaded through the X-band ground station, the GRM light curve of 15 to 600 keV shows that this burst consists of multi-pulses with a T90 of 28.6 +2.7/-9.9 s.

The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb250127C.png

The SVOM/GRM on-ground localization of this burst is (J2000): RA: 274.5 deg DEC: 25.0 deg Error: 1.94 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 159 degrees from the SVOM optical axis, and outside the ECLAIRs field of view. This burst was detected by the Count-Rate Trigger onboard ECLAIRs, as an increase in counts over background, but it was not localized by the coded-mask imaging process, which confirms that the burst occurred outside the ECLAIRs field of view.

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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