GCN Circular 37647

SVOM/GRM team: Chen-Wei Wang, Yong-Wei Dong, Jiang-Tao Liu, Shi-Jie Zheng, Jian-Chao Sun, Wen-Jun Tan, Jiang He, Min Gao, Hao-Xuan Guo, Yue Huang, Lu Li, Yong-Ye Li, Hong-Wei Liu, Xin Liu, Hao-Li Shi, Li-Ming Song, You-Li Tuo, Hao-Xi Wang, Jin Wang, Jin-Zhou Wang, Ping Wang, Rui-Jie Wang, Yu-Xi Wang, Bo-Bing Wu, Shao-Lin Xiong, Jian-Ying Ye, Yi-Tao Yin, Wen-Hui Yu, Fan Zhang, Li Zhang, Peng Zhang, Shuang-Nan Zhang, Wen-Long Zhang, Yan-Ting Zhang, Shu-Min Zhao, Xiao-Yun Zhao, Chao Zheng (IHEP), Maria-Grazia Bernardini (LUPM/INAF-OAB), Laurent Bouchet (IRAP), David Corre (CEA), Tais Maiolino (LUPM), Frédéric Piron (LUPM), Stéphane Schanne (CEA), Jingwei Wang (IAP)

SVOM JSWG: Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang (IHEP), Stéphane Basa (LAM), Shao-Lin Xiong (IHEP), Jean-Luc Attéia (IRAP), Arnaud Claret (CEA), Zi-Gao Dai (USTC), Frédéric Daigne (IAP), Jin-Song Deng (NAOC), Andrea Goldwurm (APC), Diego Götz (CEA), Xu-Hui Han (NAOC), Cyril Lachaud (APC), En-Wei Liang (GXU), Yu-Lei Qiu (NAOC), Susanna Vergani (Obs.Paris), Jing Wang (NAOC), Chao Wu (NAOC), Li-Ping Xin (NAOC), Bing Zhang (UNLV)
report on behalf of the SVOM team:

During the commissioning phase, the SVOM/GRM was triggered both in-flight and on-ground by a burst likely from the young magnetar Swift J1818.0-1607 (SVOM trigger reference: sb24081709) at 2024-08-17T21:34:06.600 UT (T0), which was also observed and located by SVOM/ECLAIRs (Miguel Llamas Lanza et al., ATel 16837).

The SVOM/GRM on-ground localization of this burst is (J2000):
RA: 276.5 deg
DEC: -16.0 deg
Error: 7.6 deg (1 sigma, statistical only)
This is consistent with the localization provided by SVOM/ECLAIRs (RA=274.530 +/- 0.048 deg, Dec = -16.052 +/- 0.043 deg). Both Swift J1818.0-1607 and Swift J1822.3-1606 located within the error area, and GRM cannot identify which source it came from independently. Combined with the results of ECLAIRs, we concluded that this burst is more likely from Swift J1818.0-1607.

The lightcurve has at least 3 pulses with high significant level, with a total duration of about 0.8 s (i.e. T0-0.3 s ~ T0+0.5 s) from 15 keV to 100 keV. The time integral spectrum of the same time interval and energy range can be adequately fitted by a single bbodyrad model with kT=3.8 (+0.37/-0.33) keV and norm= 80.7 (+52.5/-30.1), which is corresponding to a radius of 4.3 (+3.5/-2.6) or 7.3 (+5.9/-4.4) km by assuming a source distance of 4.8 or 8.1 kpc (A. Y. Ibrahim et al 2023 ApJ 943 20), respectively. The fluence of this time interval is 5.7 (+0.41/- 0.38) E-8 erg/cm2 in 15-100 keV. 

As the calibration of SVOM/GRM is still undergoing, we note that these results are preliminary. 

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.