TITLE:   GCN CIRCULAR
NUMBER:  45411
SUBJECT: GRB 260818B: SVOM/GRM observation
DATE:    26/08/20 09:16:25 GMT
FROM:    renyz16607@163.com

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

SVOM/ECLAIRs team: Sebastien Guillot (IRAP)
Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by GRB 260818B at 2026-08-18T20:11:12.700 UTC (T0), which is also detected by SVOM/ECLAIRs (Cordier et. al., GCN #45398) and HXMT (Wang et. al., GCN #45408).

With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of a pulse with a T90 of 0.16 +0.12/-0.04 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn/gcn/2026/svgrb260818B.png

In addition, the position of this burst, as determined by SVOM/ECLAIRs (RA= 44.9407, Dec=7.4547, ERR= 4.60 arcmin, GCN #45398), is located at about 29 degrees from the SVOM optical axis. 

With this localization, the time-averaged spectrum from T0-0.08 to T0+0.12 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.00 +0.22/-0.23 and the cutoff energy, parameterized as Epeak, is 70 +7/-6 keV. The event fluence (10-1000 keV) in this time interval is (2.67 +0.18/-0.16)E-07 erg/cm^2.

The localization of GRB 260818B in the 'Amati' relation diagram is shown at: 
https://www.bursthub.cn/gcn/2026/svgrb260818B_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: Yang-Zhao Ren (IHEP)(renyz@ihep.ac.cn)