TITLE: GCN CIRCULAR NUMBER: 40991 SUBJECT: GRB 250706B: TRT bright optical counterpart detection DATE: 25/07/06 18:32:19 GMT FROM: Dong Xu at NAOC/CAS <dxu@nao.cas.cn> Z.P. Zhu (NAOC), K. Noysena, S. Tinyanont, K. Chanchaiworawit (NARIT), S.Q. Jiang, X. Liu, J. An, L.B. He, D. Xu (NAOC), S.Y. Fu (HUST) report on behalf of a large collaboration: We observed the field of GRB 250706B detected by SVOM/ECLAIRs (Palmerio et al., GCN 40989) using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at New South Wales, Australia (SBO). Observations started at 17:41:58 UTC on 2025-07-06, i.e., 35.9 min after the SVOM trigger and a series of frames in the R band were obtained. An uncatalogued and varying optical transient is detected within the SVOM/MXT error circle (Palmerio et al., GCN 40989) at coordinates R.A. (J2000) = 02:44:55.45 Dec. (J2000) = -50:03:40.32 with an uncertainty of ~0.5 arcsec. It has R ~ 15.5 mag in the first R-band frame, calibrated with Legacy Survey DR-10 and not corrected for Galactic extinction. We thus think this is the optical afterglow of the GRB. View this GCN Circular online at https://gcn.nasa.gov/circulars/40991. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...