TITLE:   GCN CIRCULAR
NUMBER:  43273
SUBJECT: GRB 251222A: Mondy Optical Observations
DATE:    25/12/30 17:23:31 GMT
FROM:    Nicolai Pankov at IKI <colinsergesen@gmail.com>

N. Pankov (HSE, IKI), E. Klunko (ISTP), A. Pozanenko (IKI), A. Volnova (IKI) report on behalf of IKI-GRB-FuN:

We observed 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 R filter using the 1.5-meter AZT-33IK telescope of the Sayan Solar Observatory (Mondy). The observations began on 2025-12-25 at 14:32 UT, i.e. ~2.91 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; Freeberg et al., GCN 43229; Volnova et al., GCN 43230; Busmann et al., GCN 43233; Lee et al., GCN 43236; Maksut et al., GCN 43237; Kumar et al., GCN 43250) is detected in the co-add image of 30x120 sec. 

The observation details and preliminary photometry are presented in the table below:

Date       UT start  t-T0         Exp.    Filter   OT     Err.   UL
                     (mid, days)  (s)                           (3sigma)
2025-12-25 14:32:35  2.91441     30*120    R       21.66  0.12   22.9

The photometry was calibrated using nearby stars from USNO-B1.0 and not corrected for the Galactic extinction.