TITLE:   GCN CIRCULAR
NUMBER:  40834
SUBJECT: GRB 250625A: JinShan optical observations
DATE:    25/06/26 04:25:58 GMT
FROM:    Zipei Zhu at NAOC <zpzhu@nao.cas.cn>

Z.P. Zhu, S.Q. Jiang, 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 250625A detected by the Fermi/GBM and Swift/BAT (the Fermi GBM Team, GCN 40824; Ferro et al., GCN 40825), using the 100cm-C telescope (100C) of the JinShan project, located at Altay, Xinjiang, China. Observations started at 16:11:04 UT on 2025-06-25, i.e., 6.1 min after the BAT trigger, and a series of frames were obtained in the Sloan r-, i-, z-bands.

In the first round of r-band exposures, an uncatalogued optical source is marginally detected within the Swift/XRT enhanced error circle (Beardmore et al., GCN 40827), which is consistent with the reported optical counterpart by SAO RAS (Moskvitin et al., GCN 40828) and SVOM/VT (Xin et al., GCN 40830). The source is not detected in i-band and z-band (see also Malesani et al., GCN 40831), as well as in the second r-band exposures. Our preliminary results are summarized as follows:

T-mid (hr) | Filter | Mag
0.387        |    r    |       22.5 +/- 0.3
1.237        |    i     |       > 21.7 (5$\sigma$)
1.935        |    z    |       > 20.5 (5$\sigma$)
2.761        |    r    |       > 21.1 (5$\sigma$)

calibrated with nearby Pan-STARRS stars without the Galactic extinction correction.

We acknowledge the excellent support from T.Q. Chen and J.L. He for enabling these observations.