TITLE: GCN CIRCULAR NUMBER: 42931 SUBJECT: GRB 251201B: SVOM/VT optical fading DATE: 25/12/02 00:45:58 GMT FROM: Huali Li at at NAOC, SVOM <lhl@nao.cas.cn> H.L. Li, L.P. Xin, Y. L. Qiu, C. Wu, Z. H. Yao, Y. N. Ma, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. R. Xu, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team. SVOM/VT performed ToO observation to the field of GRB 251201B (Fermi GBM team, GCN 42923; Moss et al., GCN 42924; Osborne et al., GCN 42927; Goyal et al., GCN 42925). The observation stared at 2025-12-01T17:48:58 UTC, I.e., 1.4 minutes post trigger in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. The optical counterpart (Cerón et al., GCN 42929;Buckley et al., GCN 42930) was detected Within the error box of Swift-UVOT (Moss et al., GCN 42924) and Swift-XRT (Osborne et al., GCN 42927). The position is at R.A., Dec. = 6.39414, -44.23374 degrees, equivalent to: R.A. (J2000) = 0:25:34.59 Dec. (J2000) = -44:14:01.46 with an uncertainty of 0.5 arcsec. The measurements in AB magnitude without correction for Galactic extinction are given below: Mid_time Band Exposure Time Magnitude (AB) 1.40 hours VT_B 70 sec 18.56+/-0.03 mag 1.40 hours VT_R 70 sec 18.08+/-0.03 mag 3.04 hours VT_B 70 sec 19.46+/-0.04 mag 3.04 hours VT_R 70 sec 18.97+/-0.04 mag It was fading with a slope of about -1.0 with a single power law. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (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. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS. View this GCN Circular online at https://gcn.nasa.gov/circulars/42931. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...