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.

View this GCN Circular online at https://gcn.nasa.gov/circulars/42479. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...