TITLE: GCN CIRCULAR NUMBER: 38813 SUBJECT: GRB 250103B：SVOM/VT optical counterpart DATE: 25/01/04 02:54:27 GMT FROM: Liping Xin at NAOC, SVOM <xlp@nao.cas.cn> SVOM/VT commissioning team: Y. L. Qiu, H. L. Li, L. P. Xin, C. Wu, X. H. Han, J. Wang, W. J. Xie, H. B. Cai, Y. Xu, Y. J. Xiao, P. P. Zhang, J. S. Deng, L. Lan, X. M. Lu, R. S. Zhang, Z. H. Yao (NAOC), J. Zhang, L. J. Dan, G. Y. Zou, C. J. Wang, Y. F. Du, C. Huang (XIOPM),T. Bouchet, D. Turpin (CEA) SVOM JSWG: Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang (IHEP), Stéphane Basa (LAM), Arnaud Claret (CEA), Zi-Gao Dai (USTC), Frédéric Daigne (IAP), Jin-Song Deng (NAOC), Olivier Godet (IRAP), Andrea Goldwurm (APC), Diego Götz (CEA), Xu-Hui Han (NAOC), Cyril Lachaud (APC), En-Wei Liang (GXU), Yu-Lei Qiu (NAOC), Susanna Vergani (Obs.Paris), Jing Wang (NAOC), Chao Wu (NAOC), Li-Ping Xin (NAOC), Shao-Lin Xiong (IHEP), Bing Zhang (UNLV) report on behalf of the SVOM team: The SVOM/VT conducted a ToO follow-up observations for GRB 250103B triggered by both Swift(R. A. J. Eyles-Ferris, GCN 38796) and SVOM (T. Bouchet, GCN 38797) in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously, from 2.467 hours to 7.573 hours after the burst. An uncatalogued fading source was clearly detected within the errorbox of XRT (R. A. J. Eyles-Ferris, GCN 38796) in both VT_B and VT_R stacked images, compared to DESI DR10 catalog. The brightness of the source was estimated to be VT_B=22.1+/-0.1 mag and VT_R=21.8+/-0.1 mag in AB magnitude at the mid time of 2.79 hours post the burst, with a total exposure time of 23*100 seconds. The source is located at RA, Dec =  54.66373, -33.74927 deg, which is equivalent to: RA (J2000) : 03:38:39.30 Dec (J2000): -33:44:57.4 with an uncertainty of 0.2 arcsec. We proposed that the source is the counterpart for the gamma-ray burst. More deep follow-ups are encouraged. The magnitudes give above are not corrected for the expected Galactic foreground extinction, corresponding to a reddening of E(B-V) = 0.0067 mag in the direction of the optical counterpart (Schlafly & Finkbeiner 2011). 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/38813. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...