TITLE: GCN CIRCULAR NUMBER: 39755 SUBJECT: GRB250317B: SVOM/VT detection of an optical candidate from VHF data DATE: 25/03/17 06:48:34 GMT FROM: SVOM_group svomgroup@bao.ac.cn

J. T. Palmerio (CEA) reports on behalf of the SVOM/VT team:

After the trigger by SVOM/ECLAIRs at 2025-03-17T02:12:08 UTC (Tb), SVOM performed an automatic slew on the burst. SVOM/VT began observing the field at 2025-03-17T02:18:21, 362 seconds after the SVOM trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.

An uncatalogued source is detected within the error box of SVOM/ECLAIRs (Zhao et al. GCN 39753) using the VT VHF pipeline at ra=211.643617, dec=40.079524 (J2000), corresponding to:

RA (J2000) = 14h06m34.5s Dec (J2000) = +40:04:46.3s with an uncertainty of 0.5 arcsec.

consistent with the optical afterglow reported by Watson et al. (GCN 39754)

The source is detected in both VT_R and VT_B and was fading between the third and fourth VT observing sequences (starting at 5574 and 8474 seconds after the trigger respectively), the magnitudes are given below:

date-obs | exposure time (s) | mag(AB) | mag err | band 2025-03-17T03:45:13 | 6x50 | 17.48 | 0.02 | VT_R 2025-03-17T04:33:33 | 5x50 | 17.82 | 0.01 | VT_R 2025-03-17T03:45:13 | 6x50 | 18.89 | 0.01 | VT_B 2025-03-17T04:33:33 | 5x50 | 18.97 | 0.02 | VT_B

The source was undetected in the first two sequences (starting at 362 and 512 seconds after the trigger, respectively) and is outside the reported SVOM/MXT position.

The Space-based multi-band astronomical 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.

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