TITLE: GCN CIRCULAR NUMBER: 40041 SUBJECT: GRB 250403A: JinShan optical observations DATE: 25/04/04 09:10:46 GMT FROM: sqjiang at NAOC sqjiang@bao.ac.cn

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

We observed the field of GRB 250403A detected by SVOM/ECLAIRs (Julakanti et al., GCN 40026) and Fermi/GBM (Fermi GBM team, GCN 40025), using the 100cm-C telescope (100C) of the JinShan project, located at Altay, Xinjiang, China. Observations started at 18:03:34.11 UT on 2025-04-03, i.e., 2.80 hr after the SVOM/ECLAIRs trigger, and a series of 600 s frames were obtained in the Sloan r- and i- bands.

The optical afterglow (Starling et al., GCN 40028; Wu et al., GCN 40029; Du et al., GCN 40031; Li et al., GCN 40032; Julakanti et al., GCN 40034; Ghosh et al., GCN 40039) of GRB 250403A is clearly detected in our stacked images. Preliminary photometry results are as follows:

T_mid(hr) Filter Mag(AB) MagErr 4.06 r 19.77 0.05 4.73 i 20.11 0.06

calibrated with nearby Legacy Survey stars and not corrected for Galactic extinction.

We acknowledge the excellent support from T.Q. Chen and Z.K. Feng for enabling these observations.

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