TITLE: GCN CIRCULAR NUMBER: 42438 SUBJECT: GRB 251025B: SVOM/C-GFT optical counterpart detection DATE: 25/10/25 15:50:39 GMT FROM: SVOM_group <svomgroup@bao.ac.cn> Chao WU (NAOC), Zhe Kang (CHO), Liping Xin(NAOC), Xuhui Han(NAOC), Pinpin Zhang (NAOC), Xiaomeng Lu (NAOC), Zhenwei Li (CHO), You Lv (CHO), Ruosong Zhang (NAOC), Yujie Xiao(NAOC), Yulei Qiu(NAOC), Jing Wang (NAOC), Jinsong Deng(NAOC), Lei Huang(NAOC), Jianyan Wei (NAOC),S.Hussein (IJCLab), M. Brunet (IRAP) report on behalf of the SVOM/C-GFT team: We observed the field of GRB 251025B (sb25102502) detected by SVOM/ECLAIRs (Hussein et al. GCN 42437) with LATIOS on SVOM/C-GFT. Observations started at 2025-10-25T14:25:38 UTC, ~93 seconds after the trigger. An uncatalogued optical source compared to PanStarrs1 catalogue is detected in our images within the SVOM/MXT localization error circle at: RA (J2000) = 01h58m07.49s = 29.53123 degrees Dec (J2000) = +30h18m14.5s = 30.30405 degrees with an uncertainty of ~0.5 arcsec. The magnitudes are: |[date-obs(mid-time)] | Mid_t-T0(s) | exposure time (s) | band | mag (AB) | mag_err| |--------------------|--------------|-------------------|------|----------|--------| | 2025-10-25T14:25:38| 98 | 10 | i | 17.49 | 0.12 | The photometry was calibrated against nearby Pan-STARRS1 stars and no correction for Galactic dust extinction was applied. The source exhibited ~1 mag decline in the i band over about 10 min. We propose that it is the optical counterpart of GRB 251025B. We thank the observation assistants Hongxu Xuan and Bowen Li at Jilin observatory for their excellent support. The Chinese Ground Follow-up Telescope (C-GFT) for the SVOM mission is located at Jilin Station, Changchun Observatory, National Astronomical Observatories, CAS. It features two instruments: (1) CATCH at the Cassegrain focus with a 21 arcsec x 21 arcsec FOV for simultaneous g/r/i-band imaging, and (2) LATIOS, a 4k x 4k CMOS camera at the prime focus with a 1.28 deg x 1.28 deg FOV that images in g, r, and i bands via filter switching. View this GCN Circular online at https://gcn.nasa.gov/circulars/42438. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...