TITLE: GCN CIRCULAR NUMBER: 40041 SUBJECT: GRB 250403A: JinShan optical observations DATE: 25/04/04 09:10:46 GMT FROM: sqjiang at NAOC <sqjiang@bao.ac.cn> S.Q. Jiang, Z.P. Zhu, X. Liu, J. An, D. Xu (NAOC), S.Y. Fu (HUST), J.Z. Liu (XAO) report on behalf of a large collaboration: We observed the field of GRB 250403A detected by SVOM/ECLAIRs (Julakanti et al., GCN 40026) and Fermi/GBM (Fermi GBM team, GCN 40025), using the 100cm-C telescope (100C) of the JinShan project, located at Altay, Xinjiang, China. Observations started at 18:03:34.11 UT on 2025-04-03, i.e., 2.80 hr after the SVOM/ECLAIRs trigger, and a series of 600 s frames were obtained in the Sloan r- and i- bands. The optical afterglow (Starling et al., GCN 40028; Wu et al., GCN 40029; Du et al., GCN 40031; Li et al., GCN 40032; Julakanti et al., GCN 40034; Ghosh et al., GCN 40039) of GRB 250403A is clearly detected in our stacked images. Preliminary photometry results are as follows: T_mid(hr) Filter Mag(AB) MagErr 4.06 r 19.77 0.05 4.73 i 20.11 0.06 calibrated with nearby Legacy Survey stars and not corrected for Galactic extinction. We acknowledge the excellent support from T.Q. Chen and Z.K. Feng for enabling these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/40041. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...