TITLE: GCN CIRCULAR NUMBER: 42512 SUBJECT: GRB 251026A/EP251026a: SVOM/VT optical counterpart DATE: 25/10/29 14:28:29 GMT FROM: Huali Li at at NAOC, SVOM <lhl@nao.cas.cn> H. L. Li, C. Wu, L. P. Xin, Y. L. Qiu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, Z. H. Yao, Y. N. Ma, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. R. Xu., J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA), J.X Cao (GXU), X. Tian (GXU) report on behalf of the SVOM/VT team. SVOM/VT performed an automatic slew on the long burst triggered by SVOM/ECLAIRs (Cao et al., GCN 42454) and EP-WXT (Li et al.., GCN 42482). SVOM/VT began observing the field automatically on 2025-10-26T08:34:26, i.e., 424 sec with the slew of the platform triggered on-board, in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. With X band downlinked data, an uncatalogued and fading optical counterpart within EP-FXT's error box (Li et al., GCN 42511) was detected in both channels, compared to the PanSTARRS catalogue. The position is at R.A., Dec. = 72.653080, 51.763733 degrees, equivalent to: R.A. (J2000) = 04:50:36.74 Dec. (J2000) = +51:45:49.44 with an uncertainty of 0.5 arcsec. This location is 3.6 arcminutes from the ECLAIRs onboard position and 2.1 arcminutes from the MXTs position (Cao et al., GCN 42454). The afterglow was fading with a temporal slope of -0.7 from VT_R~19.51+/-0.05 mag, VT_B~21.35+/-0.18 mag at 445 sec, followed by a bump with the peak of VT_R~20.8+/-0.1 mag at the mid time of 3.3 hours post trigger. With the check of VT VHF data, the optical counterpart was also detected in source list and 1 bit images of both channels in Sequence 1. Our photometry was estimated in AB magnitude and not corrected for Galactic extinction. Given the heavy extinction of the Galactic plane (Av = 3.7), the burst was intrinsically optically bright. 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/42512. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...