{
  "circularId": 42766,
  "submitter": "Chenwei Wang at IHEP <cwwang@ihep.ac.cn>",
  "subject": "GRB 251118C: SVOM/GRM observation of a possible gamma-ray counterpart of EP251118a",
  "bibcode": "2025GCN.42766....1S",
  "eventId": "GRB 251118C",
  "submittedHow": "web",
  "createdOn": 1763572702158,
  "format": "text/plain",
  "body": "SVOM/GRM team: Chen-Wei Wang, Zheng-Hang Yu, Chao Zheng, Yue Huang, Shi-Jie Zheng, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)\nSVOM/ECLAIRs team: Maria-Grazia Bernardini (INAF-OAB)\nReport on behalf of the SVOM team: \n\nSVOM/GRM was triggered on-ground by a soft burst GRB 251118C at 2025-11-18T16:44:48.500 UTC (T0).  \n\nWith the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of a single pulse with a T90 of 29 +22/-12 s in the 15-5000 keV band, which is also detected by Kouns-Wind (D. Frederiks et al., GCN#42765).\n\nThe SVOM/GRM light curve can be found here:\nhttps://www.bursthub.cn//admin/static/svgrb251118C.png\n\nWe note that the time coincidence between EP251118a (S. Q. Jiang et al., GCN#42749) and GRB 251118C suggest the association of these two events. \n\nIn addition, the position of this burst, if is associated with EP251118a (RA= 123.160, DEC= 10.952, GCN#42749), is located at about 136 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view.\n\nWith this localization, the time-averaged spectrum from T0-6 to T0+17 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.86 +0.17/-0.09 and the cutoff energy, parameterized as Epeak, is 44 +37/-21 keV. The event fluence (10-1000 keV) in this time interval is (7.82 +0.84/-0.83)E-06 erg/cm^2. \n\nThe time-averaged spectrum is also well fitted by a power law function with a power law index of -2.4 +0.15/-0.16, corresponding to a fluence of (1.00 +0.12/-0.11)E-05 erg/cm^2.\n\nThe 1s peak spectrum, measured from T0-0.5 to T0+0.5 s, if fitted by a power law function with an exponential high-energy cutoff, the power law index is -1.83 +0.25/-0.12 and the cutoff energy, parameterized as Epeak, is 64 +55/-33 keV. The flux (10-1000 keV) in this time interval is (9.47 +1.30/-1.29)E-06 erg/cm^2/s.\n\nAssuming the redshift z= 1.216 (J. An et al., GCN#42756), the localization of GRB 251118C in the 'Amati' relation diagram is shown at: \nhttps://www.bursthub.cn//admin/static/svgrb251118C_amati.png\nAnd the localization in the 'Yonetoku' relation diagram is shown at: \nhttps://www.bursthub.cn//admin/static/svgrb251118C_yonetoku.png\n\nWe caution that this analysis based on the BREAKFAST framework is very preliminary [1]. Refined analysis will be reported later.\n\nThe 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.\n\nThe SVOM point of contact for this burst is: Chen-Wei Wang (IHEP)(cwwang@ihep.ac.cn)\n\n[1] Chen-Wei Wang et al. arXiv: 2510.15816 [astro-ph.HE] https://arxiv.org/abs/2510.15816\n"
}