TITLE: GCN CIRCULAR NUMBER: 37565 SUBJECT: EP240919a: JinShan optical upper limits DATE: 24/09/20 09:08:37 GMT FROM: Dong Xu at NAOC/CAS dxu@nao.cas.cn

S.Q. Jiang, J. An, X. Liu, S.Y. Fu, Z.P. Zhu, D. Xu (NAOC), J.Z. Liu (XAO) report on behalf of a large collaboration:

We observed the field of EP240919a detected by EP/WXT (Liang et al., GCN 37561) and likely Fermi/GBM (Liang et al., GCN 37563), using the 100cm-B telescope (100B) of the JinShan project, located at Altay, Xinjiang, China. Observations started at 15:08:25 UT on 2024-09-19, i.e., 14.71 min after the EP/FXT trigger, and a series of frames were obtained in the Sloan r- and i- bands.

No new optical source is detected within the EP/FXT error circle (Liang et al., GCN 37561), down to the following 5-sigma upper limits:

|T_mid(min)|filter|U.L.(AB)| |47.46 |r| 20.0| |35.79 |i| 19.5|

calibrated with nearby PanSTARRS stars and not corrected for Galactic extinction.

We acknowledge the excellent support from X. Yao, S.W. Luo, and Z.K. Feng for enabling these observations.

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