TITLE: GCN CIRCULAR NUMBER: 42418 SUBJECT: EP251023b/GRB 251023B: JinShan optical observation DATE: 25/10/24 08:38:09 GMT FROM: L. B. He at NAOC <helb@bao.ac.cn> L.B. He, J. An, S.Q. Jiang, X. Liu, Z.P. Zhu, D. Xu (NAOC), S.Y. Fu, A.D. Zhu, L. Lei, H.Z. Wu (HUST), J.Z. Liu (XAO) report on behalf of a large collaboration: We observed the field of EP251023b/GRB 251023B (Kang et al., GCN 42392; Nakahira et al., GCN 42402; Wu et al., GCN 42415) using the 100C telescope of the JinShan project, located at Altay, Xinjiang, China. Observations started at 2025-10-23 16:01:41 UT, i. e. 7.3 hours after the trigger. The previously reported optical counterpart (Mandarakas et al., GCN 42404) was detected in our stacked z-band frame with a magnitude z~21.4 at a mid-time 9.75 hrs post-burst. The magnitude was callibrated with nearby PanSTARRS1 dr2 catalog without the Galactic extinction correction. We acknowledge the excellent support from T.Q. Chen and J.F. Zhang for enabling these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/42418. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...