TITLE: GCN CIRCULAR NUMBER: 41679 SUBJECT: GRB 250903A: TRT optical counterpart detection DATE: 25/09/03 18:46:15 GMT FROM: J. An <jiean0813@foxmail.com> J. An (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), S.Y. Fu (HUST), L.B. He, X. Liu, Z.P. Zhu, S.Q. Jiang, D. Xu (NAOC) report on behalf of a large collaboration: We observed the field of GRB 250903A detected by SVOM/ECLAIRs (Maiolino et al., GCN 41677), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at New South Wales, Australia (SBO). Observations started at 17:34:50 UTC on 2025-09-03, i.e., ~10 min after the SVOM/ECLAIRs trigger. A series of R-band frames were obtained. An uncatalogued and varying optical source is detected within the SVOM/ECLAIRs error circle (Maiolino et al., GCN 41677), at coordinates R.A. (J2000) = 23:47:11.67 Dec. (J2000) = -75:58:00.21 with an uncertainty of ~ 0.8 arcsec. Preliminary photometry shows the source has R ~ 19.0 mag at 13.35 min post-trigger, calibrated with Legacy Survey DR10 catalog and not corrected for Galactic extinction. We think the source is the optical counterpart of GRB 250903A. View this GCN Circular online at https://gcn.nasa.gov/circulars/41679. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...