TITLE: GCN CIRCULAR NUMBER: 44424 SUBJECT: GRB 260428A: SVOM/GRM observation of a possible short burst with extended emission DATE: 26/04/29 15:26:34 GMT FROM: Chenwei Wang at IHEP <cwwang@ihep.ac.cn> SVOM/GRM team: Chen-Wei Wang, Chao Zheng, Zheng-Hang Yu, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP) SVOM/ECLAIRs team: Olivier GODET (IRAP) Report on behalf of the SVOM team: SVOM/GRM was triggered in-flight by a burst GRB 260428A (SVOM trigger reference: sb26042803) at 2026-04-28T14:20:37.000 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN#44415), AstroSat (A. Goyal et al., GCN#44421) and GECAM-B (Wang et al., GCN#44423). With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of mutli-pulses hard spikes followed by a soft weak extended emission with a T90 of 4.1 +3.1/-2.1 s in the 15-5000 keV band. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb260428A.png In addition, the position of this burst, as determined by Fermi/GBM (RA = 277.0 deg, Dec = 1.9 deg, Err= 2.1 deg, GCN#44415), is located at about 100 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. With this localization, the time-averaged spectrum from T0-1.4 to T0+8.4 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.11 +0.33/-0.25 and the cutoff energy, parameterized as Epeak, is 1380 +159/-66 keV. The event fluence (10-1000 keV) in this time interval is (4.82 +0.83/-0.79)E-06 erg/cm^2. GRB 260428A falls into the Type I region in the 'Amati' relation diagram, as shown at: https://www.bursthub.cn//admin/static/svgrb260428A_amati.png With the brightness of the prompt emission, the predicted soft X-ray afterglow [1] is shown at: https://www.bursthub.cn//admin/static/svgrb260428A_xag_pre.png 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 997 353 https://doi.org/10.3847/1538-4357/ae2eae View this GCN Circular online at https://gcn.nasa.gov/circulars/44424. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...