TITLE: GCN CIRCULAR NUMBER: 38639 SUBJECT: GRB 241218A: Mondy AZT-33IK optical upper limit DATE: 24/12/19 19:27:24 GMT FROM: Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com> N. Pankov (HSE, IKI), A. Pozanenko (IKI), E. Klunko (ISTP) report on behalf of IKI-GRB-FuN: We performed optical observations of the field of GRB 241218A (Williams et. al, GCN 38601; Beardmore et. al, GCN 38604; Strausbaugh et. al, GCN 38605; Maksut et. al, GCN 38611; D'Elia et. al, GCN 38614; Brivio et. al, GCN 38620; Gupta et. al, GCN 38629; Fu et. al, GCN 38630; Siegel et. al, GCN 38634) in the R filter with AZT-33IK of Sayan observatory (Mondy). The observations began on 2024-12-18 20:52 UT, i.e. ~0.88 days since Swift trigger. We do not detect an optical counterpart inside the XRT enhanced localization (Beardmore et. al, GCN 38604) in the stacked image of 58x60 sec. The preliminary photometry is given below: Date UT start t-T0 Exp. Filter UL(3sigma) (mid, days) (s) 2024-12-18 20:53:01 0.88415 58*60 R 20.8 The magnitudes were calibrated using nearby stars from USNO-B1.0 catalog (R2 magnitudes) and are not corrected for the Galactic extinction. The results are consistent with non-detections reported by other groups. View this GCN Circular online at https://gcn.nasa.gov/circulars/38639. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...