TITLE:   GCN CIRCULAR
NUMBER:  42477
SUBJECT: GRB 251017A: INASAN-Kislovodsk optical observations
DATE:    25/10/27 13:36:52 GMT
FROM:    Alina Volnova at IKI RAS <alinusss@gmail.com>

A. Volnova (IKI), A.N. Tarasenkov (INASAN), A. Pozanenko (IKI), N. Pankov (HSE) report on behalf of IKI GRB-FuN:

We observed the field of the Swift GRB 251017A (Gupta et al., GCN 42322) with RC500 telescope of the INASAN-Kislovodsk observatory. We took several 120-seconds frames in the R-band starting Oct. 17, 2025, 15:09:15 (UT). In the stacked image the optical counterpart (Perez-Garcia et al., 42323; Antier et al., 42325; Sasada et al., 42329; Lipunov et al., 42330; Fu et al., 42331; Mo et al., 42336; Adami et al., 42339; Xin et al., 42341; Izzo et al., 42350; Shilling et al., 42355; Akl et al., 42368; Midavaine et al., 42370) is clearly detected. Preliminary photometry is the following:

Date        UT start  t-T0       Exp.     Filter  OT     Err.  UL
                     (mid,days)  (n*s)                        (3sigma)
2025-10-17  15:09:15  0.34348    100*120  R       21.45  0.16  22.0

The photometry is based on several nearby PS1 stars (R-magnitudes Lupton's transformation) and is not corrected for the Galactic extinction.