TITLE: GCN CIRCULAR NUMBER: 39636 SUBJECT: GRB 250308A: SVOM/GRM detection of QPO in the tail emission DATE: 25/03/09 16:00:23 GMT FROM: Chenwei Wang at IHEP cwwang@ihep.ac.cn

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

SVOM/ECLAIRs team: Nicolas Dagoneau (CEA), Maria-Grazia Bernardini (INAF-OAB), Jean-Luc Atteia (IRAP), Frédéric Piron (LUPM)

Report on behalf of the SVOM team:

The bright short burst, GRB 250308A (GCN #39632), consists of a narrow spike followed by a short tail emission (from T0-0.15 to T0+0.15 s), which shows a feature of QPO, with a center frequency at approximately 31 Hz.

The on-ground localization of this burst is (J2000): RA: 170.9 deg DEC: 25.6 deg Error: 5.2 deg (1sigma, statistical only) We caution that the calibration of SVOM/GRM is undergoing and this localization is subject to systematic errors.

With this localization, the time-averaged spectrum from T0-0.3 to T0+0.2 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.28 +/- 0.04 and the cutoff energy, parameterized as Epeak, is 452 +/- 56 keV. The event fluence (10-1000 keV) in this time interval is (4.9 +/- 0.2)E-06 erg/cm^2.

The feature of QPO in the tail emission and the spectrum parameters are consistent with the scenario of magnetar giant flare. Follow-up observations are strongly encouraged.

We note that these results are preliminary. Refined analysis will be reported later.

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/GRM point of contact for this burst is: Chen-Wei Wang (IHEP) (cwwang@ihep.ac.cn)

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