GRB 240821A : a likely short GRB detected by SVOM/ECLAIRs

During the commissioning phase, the SVOM/ECLAIRs telescope triggered and located GRB 240821A (sb24082109) a bright GRB at 2024-08-21T18:35:50.150 UT (T0), which was also detected by SVOM/GRM (GCN in preparation) and Fermi/GBM (GCN 37219). The burst was detected by both the on-board Count-Rate Trigger (CRT) and Image Trigger (IMT) and 40 alerts were received on the ground with low-latency trough the SVOM VHF Alert Network. The best detection is obtained by IMT with a signal to noise ratio of 29.7 in the 8-120 keV energy band on a time window of 81.92 s starting at T0. The sub-image received shows a clear point-like source. The transient location is RA, Dec = 354.23, -10.18 (J2000). This is about 23 degrees from the ECLAIRs optical axis, in the partially coded field of view. The statistical uncertainty on this position is 3 arcminutes, to which we recommend to add 10 arcminutes of systematic uncertainty. This position is 7° from the Fermi localization (GCN 37219) and thus consistent with it. The light-curve of the GRB shows a bright spike, followed by fainter emission during about 100 seconds, suggesting a short GRB with extended emission. SVOM did not slew to the burst since automated slewing is not yet enabled. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), French Space Agency (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. ECLAIRs was developed jointly by APC, CEA, CNES and IRAP. The SVOM point of contact for this burst is Sebastien Guillot: sebastien.guillot@irap.omp.eu