TITLE: GCN CIRCULAR NUMBER: 40561 SUBJECT: GRB 250521D: SVOM/GRM analysis suggests a possible “long” duration type I burst DATE: 25/05/28 10:15:39 GMT FROM: Chenwei Wang at IHEP cwwang@ihep.ac.cn
SVOM/GRM team: Chen-Wei Wang, Wang-Chen Xue, Wen-Jun Tan, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)
SVOM/ECLAIRs team: Marius Brunet (IRAP), Frédéric Piron (LUPM)
report on behalf of the SVOM team:
SVOM/GRM was triggered in-flight by a long burst GRB 250521D at 2025-05-21T22:21:56.3 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #40516).
The real-time alert data and light curves of SVOM/GRM were downlinked to the ground through the VHF system with low latency. The GRM light curve shows that this burst consists of multiple episodes, including a possible precursor, a main emission followed by an possible extended emission, with a T90 of 12.1 +1.7/-5.4 s in the 15-5000 keV band.
The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb250521D.png
In addition, the position of this burst, as determined by Fermi/GBM (RA= 281.6, Dec= 21.3, GCN #40516), is located at about 80 degrees from the SVOM optical axis.
With this localization, the time-averaged spectrum from T0–2 s to T0+15 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.9 +/-0.1 and the cutoff energy, parameterized as Epeak, is 890 +302/-194 keV. The event fluence (10-1000 keV) in this time interval is (2.95 +0.76/-0.62)E-07 erg/cm^2. Thus GRB 250521D is consistent with Type I GRBs in the 'Amati' relation diagram although the duration is much longer than 2 seconds, as shown at: https://www.bursthub.cn//admin/static/grb250521D_amati.png
Since the special location of this GRB in 'Amati' relation diagram, as well as the “Type IL” pattern of lightcurve, which is similar with other “long” duration type I GRBs (i.e. GRB 211211A and GRB 230307A), a possible merger origin is suggested [1,2]. Further 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)
[1] Chen-Wei Wang et al. ApJ 979 73 https://doi.org/10.3847/1538-4357/ad98ec [2] Wen-Jun Tan et al. arXiv:2504.06616 [astro-ph.HE] https://doi.org/10.48550/arXiv.2504.06616
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