TITLE: GCN CIRCULAR NUMBER: 39973 SUBJECT: GRB 250320C : SVOM detection of a burst through offline search DATE: 25/03/31 14:25:24 GMT FROM: SVOM_group svomgroup@bao.ac.cn

M. Brunet (IRAP), S. Schanne (CEA), J. Atteia (IRAP), C. W. Wang, Y. Huang (IHEP), O. Godet (IRAP), D. Turpin (CEA)

report on behalf of the SVOM team:

The SVOM/ECLAIRs telescope detected a transient source, labelled GRB 250320C, starting at 2025-03-20T17:39:40.0 UTC (Tb), through an offline search with the event-by-event data downloaded through the X-band ground station.

The burst was detected by several methods and within several energy ranges and timescales. The best detection is obtained by the image trigger with a signal-to-noise ratio of 7.7 within 4-120 keV over a time window of 40.96 seconds starting at 2025-03-20T17:40:39.4 UTC.

The lightcurve shows a first episode, starting at Tb, composed of a single peak that lasted around 5s and a second episode 1 minute later composed of multiple peaks, that lasted around 40s.

The localization of the source is RA, Dec = 170.930, 40.677 (J2000). The uncertainty on this position is 11.6 arcminutes at 90% C.L.

We notice the presence of several quasars in this error box, among which the closest is SDSS J112340.66+404052.0 located at 0.5 arcmin. However the lightcurve is more suggestive of a GRB.

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.

The SVOM point of contact for this event is: Marius Brunet (IRAP) marius.brunet@irap.omp.eu. Please contact him by email if you require additional information regarding the SVOM follow-up of this burst.

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