TITLE:   GCN CIRCULAR
NUMBER:  40807
SUBJECT: GRB 250620C: SVOM/GRM observation
DATE:    25/06/22 05:55:58 GMT
FROM:    Chenwei Wang at IHEP <cwwang@ihep.ac.cn>

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

SVOM/ECLAIRs team: Ulysse Jacob (LUPM)

Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by a bright burst GRB 250620C (SVOM trigger reference: sb25062006) at 2025-06-20T16:07:10.100 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN#40794).

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 with a T90 of 30.1 +/-1.5 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb250620C.png

In addition, the position of this burst, as determined by Fermi/GBM (RA= 142.6, DEC= 55.0, GCN#40794), is located at about 161 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view.

With this localization, the time-averaged spectrum from T0-1 to T0+40 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.22 +0.13/-0.12 and the cutoff energy, parameterized as Epeak, is 502 +74/-55 keV. The event fluence (10-1000 keV) in this time interval is (9.15 +0.26/-0.28)E-07 erg/cm^2. 

The localization of GRB 250620C in the 'Amati' relation diagram is shown at: 
https://www.bursthub.cn//admin/static/grb250620C_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: Chen-Wei Wang (IHEP)(cwwang@ihep.ac.cn)