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.

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