TITLE: GCN CIRCULAR NUMBER: 40252 SUBJECT: GRB 250424A: SVOM/GRM observation of a long burst DATE: 25/04/26 13:30:39 GMT FROM: zhangjinpeng@ihep.ac.cn

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

SVOM/ECLAIRs team: Jean-Luc Atteia (IRAP), Olivier Godet (IRAP), Stéphane Schanne (CEA), Frédéric Daigne (IAP)

Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by GRB 250424A (SVOM trigger reference: sb25042401) at 2025-04-24T06:52:08.000 (T0). This burst was also detected by Swift (Cenko et al., GCN 40224), AstroSat CZTI (Harsha et al., GCN 40231), Konus-Wind (Ridnaia et al., GCN 40243) and EIRSAT-1 GMOD (McKenna et al., GCN 40249).

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 18.8 +1.0/-1.0 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb250424A.png

In addition, the position of this burst, as determined by Swift (Cenko et al., GCN 40224, RA: 217.50010 deg, DEC: -35.02493 deg, Error: 1.9 arcseconds), is located at about 63.7 degrees from the SVOM optical axis, and outside the ECLAIRs field of view.

The time-averaged spectrum from T0-5 to T0+18 s is best fitted by Band function. The alpha is -1.14 +/- 0.06, the beta is -2.44 +/- 0.08, and the cutoff energy, parameterized as Epeak, is 89 +/- 5 keV. The event fluence (10-1000 keV) in this time interval is (6.63 +/- 0.12)E-05 erg/cm^2. With a redshift of 0.310 (Saccardi et al., GCN 40228), GRB 250424A is consistent with Type II GRBs in the 'Amati' relation diagram, as shown at: https://www.bursthub.cn//admin/static/grb250424A_amati.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/GRM point of contact for this burst is: Jin-Peng Zhang (IHEP) (zhangjinpeng@ihep.ac.cn)

View this GCN Circular online at https://gcn.nasa.gov/circulars/40252. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...