TITLE: GCN CIRCULAR NUMBER: 39818 SUBJECT: GRB 250321B: SVOM detection of a sub-threshold transient event DATE: 25/03/21 16:40:37 GMT FROM: SVOM_group svomgroup@bao.ac.cn

S. Schanne (CEA), M. Brunet (IRAP), D. Turpin (CEA), T. Maiolino, F. Piron (LUPM), O. Godet, J-L. Atteia (IRAP)

report on behalf of the SVOM mission team

The SVOM/ECLAIRs telescope detected a transient source appearing below the onboard trigger thresholds, labelled GRB 250321B starting at 2025-03-21T03:36:28 UTC (Tb).

The information of this sub-threshold source was received on the ground with low-latency by the SVOM VHF Alert Network. After arrival of the X-band data the source detection is confirmed.

The burst was detected below the thresholds applied onboard by both the Count-Rate Trigger (CRT) and Image Trigger (IMT). It has been reported in the log messages of the onboard trigger downloaded via X-band. The best detection is found by IMT with a signal-to-noise ratio of 6.45 in the 8-120 keV energy band over a time window of 20.48 s starting at Tb. After reprocessing the photon by photon data in the onboard trigger replay with lower thresholds, as well as the on-ground offline trigger, a point-like source is confirmed in the reconstructed sky images.

The localization of the source is RA, Dec = 169.687, 39.560 (J2000). The uncertainty on this position is about 12 arcminutes at 90% C.L.

We notice the presence of several quasars in this error box, among which the closest is SDSS J111850.26+393258.6 located at 1.2 arcmin.

We encourage follow-up observation of this source candidate.

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.

The point of contact for this event is Stéphane Schanne (s.schanne AT cea.fr) by email if you require additional information.

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