GRB 240809A: JinShan optical observations

We observed the field of GRB 240809A detected by Swift (Evans et al. GCN 37110) using the 100C telescope of the JinShan project located at Altay, Xinjiang, China. A series of 200 s and 300 s frames were obtained in the Sloan g-, r-, i- and z- bands, starting at 15:08:20.24 UT on 2024-08-09, i.e., 6.63 hr after the Swift/BAT trigger. The optical afterglow (Evans et al. GCN 37110; Dubay et al. GCN 37114) of the burst is clearly detected in our stacked images. Preliminary photometric results are as follows: ----------------------------------------------------------------- Tmid-T0(hr) Exp(s) Filter Mag+/-MagErr ----------------------------------------------------------------- 7.404 3x200 g 20.05 +/- 0.05 7.578 3x200 r 19.86 +/- 0.09 6.758 3x300 i 19.49 +/- 0.04 7.100 5x300 z 19.47 +/- 0.08 ----------------------------------------------------------------- calibrated with nearby PanSTARRS stars and not corrected for Galactic extinction. We acknowledge the excellent support from S.W Luo, M.M. Yang, Z. K. Feng, Q. C. Zhao and L.F. Huo for enabling these observations.