TITLE: GCN CIRCULAR NUMBER: 45336 SUBJECT: GRB 260809A: SVOM/VT optical counterpart DATE: 26/08/09 22:39:24 GMT FROM: Huali Li at at NAOC, SVOM <lhl@nao.cas.cn> H. L. Li, C. Wu, L. P. Xin, Y. L. Qiu, Y. N. Ma, Z. H. Yao, J. R. Xu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. T. Palmerio (CEA/Irfu), F. Piron (LUPM) report on behalf of the SVOM/VT team. SVOM/VT performed observations with automatic slew for GRB 260809A, which was triggered by SVOM/ECLAIRs (sb26080907, Brunet et al., GCN 45331) and also detected by SVOM/MXT (Brunet et al., GCN 45331). The observation started at 2026-08-09T15:01:31 UTC, i.e., 238 seconds post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously. An uncatalogued optical counterpart was detected within the error circles of MXT (Brunet et al., GCN 45331)and ECLAIRs (Brunet et al., GCN 45331), compared with the Legacy Survey. The position is at R.A., Dec. = 356.962498, 21.037855 degrees, equivalent to: R.A. (J2000) = 23:47:50.99 Dec. (J2000) = +21:02:16.28 with an uncertainty of 0.5 arcsec. The following measurements are in the AB magnitude and are not corrected for Galactic extinction: Mid time | Exposure Time | Band | Brightness 263 s | 50 s | VT_B | 21.0+/-0.2 263 s | 50 s | VT_R | 18.8+/-0.1 3.02 h | 25*100 s | VT_B | 23.2+/-0.2 3.02 h | 27*100 s | VT_R | 21.5+/-0.1 The source showed a slow brightening trend in both channels during our early observations taken within 600 seconds post trigger. It then faded with a temporal decay slope of -1.2 since 1.02 hr. Given the fading of the source in our observations, we propose that it is the optical counterpart of the burst. 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/45336. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...