TITLE: GCN CIRCULAR NUMBER: 37547 SUBJECT: EP240918a: JinShan optical upper limits DATE: 24/09/18 16:46:22 GMT FROM: Dong Xu at NAOC/CAS dxu@nao.cas.cn

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

We observed the field of EP240918a detected by EP/WXT (Zhang et al., GCN 37541), using the 50cm-A/B and 100cm-C telescopes (50A, 50B and 100C) of the JinShan project, located at Altay, Xinjiang, China. Observations started at 13:38:24 UT on 2024-09-18, i.e., 2.23 hr after the EP trigger, and a series of frames were obtained in the Sloan g-, r-, i- and z- bands.

No new optical source is detected within the EP/FXT error circle (Zhang et al., GCN 37541), down to the following 5-sigma upper limits:

T_mid (UT) | T_mid (hr) | Telescope | filter | U.L. (AB) ---------------------------------------------------- 2024-09-18T13:53:45.0 | 2.49 | 50B | z | 17.1 2024-09-18T13:55:53.5 | 2.52 | 50A | r | 19.3 2024-09-18T14:24:52.5 | 3.00 | 50B | i | 18.2 2024-09-18T14:26:51.0 | 3.04 | 50A | g | 19.2 2024-09-18T15:52:16.8 | 4.46 | 100C | r | 21.2

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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