TITLE: GCN CIRCULAR NUMBER: 20307 SUBJECT: GRB 161220A: Swift/UVOT Upper Limits DATE: 16/12/20 17:10:33 GMT FROM: Frank Marshall at GSFC F. E. Marshall (NASA/GSFC) and V. D'Elia (ASDC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 161220A 78 s after the BAT trigger (D'Elia et al., GCN Circ. 20302). No optical afterglow consistent with the XRT position (D'Elia et al. GCN Circ. 20302) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white_FC 78 228 147 >20.5 white 78 1012 295 >21.0 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.01 in the direction of the burst (Schlegel et al. 1998).