TITLE: GCN CIRCULAR NUMBER: 41458 SUBJECT: GRB 250819A: SVOM/GRM observation of a possible “long” duration type I burst DATE: 25/08/21 06:02:19 GMT FROM: Chenwei Wang at IHEP cwwang@ihep.ac.cn

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

SVOM/ECLAIRs team: Tais Maiolino (LUPM), Stéphane Schanne (CEA)

Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by a burst GRB 250819A (SVOM trigger reference: sb25081903) at 2025-08-19T17:49:46.000 UTC (T0), which is also detected by Insight-HXMT/HE (Wang et al., GCN#41457).

With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of multi-pulses with a T90 of 7.4 +3.2/-2.0 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb250819A.png

The SVOM/GRM on-ground localization of this burst is (J2000): RA: 354.6 deg DEC: -18.8 deg Error: 12.6 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 32 degrees from the SVOM optical axis, which is inside the ECLAIRs field of view and consistent with a sub-threshold detection of ECLAIRs. The preliminary ECLAIRs localization is RA: 338.3 deg, DEC: -19.8 deg, with an error estimated as 30 arcmin, close to the edge of ECLAIRs field of view.

With the localization from ECLAIRs, the time-averaged spectrum from T0-1.5 to T0+8.0 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.98 +0.13/-0.10 and the cutoff energy, parameterized as Epeak, is 1740 +1250/-750 keV. The event fluence (10-1000 keV) in this time interval is (4.11 +0.35/-0.34)E-06 erg/cm^2.

Thus GRB 250819A is well consistent with Type I GRBs in the 'Amati' relation diagram, as shown at: https://www.bursthub.cn//admin/static/grb250819A_amati.png

Based on the location of this GRB in 'Amati' relation diagram, a possible merger origin is suggested. A SVOM ToO has been programmed. Follow-up observations are strongly encouraged.

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)

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