TITLE:   GCN CIRCULAR
NUMBER:  42479
SUBJECT: GRB 251017A: Mondy AZT-33IK and Koshka Zeiss-1000 optical observations
DATE:    25/10/27 13:47:19 GMT
FROM:    Alina Volnova at IKI RAS <alinusss@gmail.com>

A. Volnova (IKI), E. Klunko (ISTP), A. Novichonok (Petrozavodsk State University, KIAM), I. Nikolenko (INASAN), A. Pozanenko (IKI), N. Pankov (HSE, IKI), report on behalf of IKI-GRB-FuN:

We observed 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; Volnova et al., 42477) of the Swift GRB 251017A (Gupta et al., GCN 42322) in the R-filter with 1.5-meter AZT-33IK telescope of Sayan Solar Observatory (Mondy) on Oct. 17,18,23, 2025, and in the Rb filter with 1-meter Zeiss-1000 telescope of the Koshka Observatory (INASAN) on Oct. 20, 2025. Preliminary photometry and observation details are the following:

Date        UT start  t-T0       Exp.    Filter  OT     Err.  UL      Site/Instrum
                     (mid,days)  (n*s)                       (3sigma)
2025-10-17  11:25:01  0.14333    20*120  R       20.41  0.07  21.8    Mondy/AZT-33IK
2025-10-17  12:49:15  0.19065    26*120  R       20.88  0.07  22.3    Mondy/AZT-33IK
2025-10-18  11:40:09  1.14891    35*120  R       21.79  0.13  22.5    Mondy/AZT-33IK
2025-10-20  16:36:07  3.37307    40*180  R       n/d    n/d   21.1    Koshka/Zeiss-1000
2025-10-23  13:18:38  6.22425    45*120  R       n/d    n/d   22.5    Mondy/AZT-33IK

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