TITLE: GCN CIRCULAR NUMBER: 40710 SUBJECT: EP250612a/GRB 250612B: JinShan optical upper limit DATE: 25/06/12 17:24:35 GMT FROM: liuxing@nao.cas.cn X. Liu, Z.P. Zhu, J. An, S.Q. Jiang, D. Xu (NAOC), S.Y. Fu(HUST), J.Z. Liu (XAO) report on behalf of a large collaboration: We observed the field of EP250612a detected by EP/WXT (Hua et al., GCN 40700), which is very likely GRB 250612B by Fermi/GBM (Preis & Greiner, GCN 40701; Fermi GBM tea, GCN 40702), using the 100cm-C telescope (100C) of the JinShan project, located at Altay, Xinjiang, China. Observations started at 15:45:13 UT on 2025-06-12, i.e., 3.29 hr after the EP trigger, and 18 x 200 s frames were obtained in the Sloan i-band. No new optical source is detected within the EP/FXT error circle (Hua et al., GCN 40700), down to a 3-sigma upper limit of i ~ 20.2 mag (AB), calibrated with nearby PanSTARRS stars and not corrected for Galactic extinction. We acknowledge the excellent support from T.Q. Chen for enabling these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/40710. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...