TITLE: GCN CIRCULAR NUMBER: 43226 SUBJECT: GRB 251222A: ZTSH BVRI optical observations DATE: 25/12/24 08:57:11 GMT FROM: Nicolai Pankov at IKI <colinsergesen@gmail.com> N. Pankov (HSE, IKI), V. Rumyantsev (CrAO), A. Pozanenko (IKI), A. Volnova (IKI) report on behalf of IKI-GRB-FuN: We performed optical observations of the field of GRB 251222A (The Fermi GBM team, GCN 43186; Yang et. al, GCN 43188; Preis & Greiner, GCN 43190; Evans, GCN 43191; Beardmore et. al, GCN 43213; Frederiks et. al, GCN 43214; Guo et. al, GCN 43218; Neights et. al, GCN 43222) at z = 3.171 (Saccardi et. al, GCN 43204) in the BVRI filters using the 2.6-meter ZTSh telescope of CrAO. The observations began in the I-filter on 2025-12-22 at 21:15 UT, i.e. 0.173 days since trigger. The optical counterpart (Palmerio et. al, GCN 43189; An et. al, GCN 43194; Calapai et. al, GCN 43195; Durbak et. al, GCN 43197; Li et. al, GCN 43203; Saccardi et. al, GCN 43208; Gupta et. al, GCN 43207; Saccardi et. al, GCN 43204; Kumar et. al, GCN 43209; Pérez-Fournon et. al, GCN 43212; Gupta et al., GCN 43224; Globus et al., GCN 43225) is detected in the majority of individual V,R images. The observation properties and the preliminary photometry of a V,R-filter image pair are presented in the table below: Date UT start t-T0 Exp. Filter OT Err. UL (mid, days) (s) (3sigma) 2025-12-22 21:36:41 0.188142 120 V 19.14 0.08 20.8 2025-12-22 21:38:48 0.189615 120 R 18.77 0.04 21.4 The photometry was calibrated using nearby stars from USNO-B1.0 and not corrected for the Galactic extinction. A power law fit to the R-filter light curve gives a temporal index of -0.64 +- 0.08. View this GCN Circular online at https://gcn.nasa.gov/circulars/43226. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...