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.