GRB 241209B: SVOM/ECLAIRs and GRM detection of a long burst

During the commissioning phase, the SVOM/ECLAIRs telescope triggered and located the long duration GRB 241209B (sb24120901) at 2024-12-09 03:56:20 UT (Tb) which was also detected by SVOM/GRM. The following ECLAIRs trigger information was received on the ground with low-latency by the SVOM VHF Alert Network. The burst was detected by both the on-board Count-Rate Trigger (CRT) and Image Trigger (IMT) and 13 alerts were received. The best detection is obtained by CRT with a signal-to-noise ratio of 20.01 in the 8-120 keV energy band over a time window of 1.2 s starting at Tb. The light curve shows one main pulse. The burst duration is 3.7 -0.2 / +1.1 s (5-120 keV). The localization of the best alert is RA, Dec = 194.983, 76.222 (J2000). The statistical uncertainty on this position is 4.2 arcminutes, to which we recommend adding 2 arcminutes of systematic uncertainty in quadrature. SVOM/GRM was also triggered in-flight by this burst (SVOM trigger reference: sb24120901) at 2024-12-09T03:56:20.000 UTC (T0). With the VHF data, the GRM light curve shows that this burst consists of one main pulse with a duration of 3.7 +/- 0.7 s (15-550 keV). The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb241209B.png SVOM immediately slew to the ECLAIRs error box and MXT started to observe the field of GRB 241209A at 2024-12-09T04:43 UT in an automatic way, about 50 minutes after ECLAIRs detection. The observation start delay is due to Earth occultation. The analysis of the prompt VHF data shows no clear evidence of an afterglow candidate in the MXT data. Further analysis will be done once the X-band telemetry data are received. Observations by other telescopes are encouraged. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), 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. GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. MXT was developed jointly by CNES, CEA, IJCLab, University of Leicester and MPE. The SVOM point of contact for this burst is: xiewj@bao.ac.cn