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.

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