TITLE: GCN CIRCULAR NUMBER: 43180 SUBJECT: GRB 251221A: UAFO and Mondy optical upper limit DATE: 25/12/22 12:03:11 GMT FROM: Alina Volnova at IKI RAS <alinusss@gmail.com> A. A. Volnova (IKI), A. Kochergin (UAFO IAA), E. Klunko (ISTP), A. S. Pozanenko (IKI), N. S. Pankov (HSE, IKI) report on behalf of IKI-GRB-FuN: We observed the field of GRB 251221A (Fermi GBM team, GCN 43164; Ambrosi et al., GCN 43166) with the RC-500 (0.5m) telescope of Ussuriysk Astrophysical Observatory (UAFO) and with the AZT-33IK 1.5m telescope of the Sayan Solar Observatory (Mondy) taking several frames in R-band. The observations started on Dec. 21 at UT 15:18:04 with RC-500 and at UT 17:25:26, i.e., ~ 0.4-0.5 days after the Swift/BAT trigger. In the stacked images in the Swift/XRT error circle (Goad et al., GCN 43175) we do not detect the optical counterpart reported previously by Rakotondrainibe et al. (GCN 43168). Preliminary photometry and observational details are the following: Date UT start t-T0 Exp. Filter Obj. Err. UL Site/Telescope fwhm" (mid,days) (n*s) (3sigma) 2025-12-21 15:18:04 0.42900 60*60 R n/d n/d 19.7 UAFO/RC-500 2.7 2025-12-21 17:25:26 0.51314 18*120 R n/d n/d 22.1 Mondy/AZT-33IK 2.6 The photometry is based on several nearby stars from the USNO-B1 catalogue (R2 magnitudes) and is not corrected for the Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/43180. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...