TITLE: GCN CIRCULAR NUMBER: 20339 SUBJECT: GRB 161224A: Swift/UVOT Upper limits DATE: 16/12/26 12:12:17 GMT FROM: Paul Kuin at MSSL N. P. M. Kuin (UCL-MSSL) and A. Cholden-Brown (PSU) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 161224A 28.3ks after the BAT trigger after a Sun constraint (Cholden-Brown et al., GCN Circ. 20335). At the position of the XRT source (Cholden-Brown and Evans, GCN Circ. 20338) no uv-optical source was found to the upper limits below. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag u 44266.0 44786.3 508 >19.99 v 28322.2 50899.7 1770 >19.90 uvw1 35223.6 41032.4 786 >19.91 uvw2 46285.0 46772.6 480 >19.88 uvm2 29234.3 35216.6 1198 >20.48 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.096 in the direction of the burst (Schlegel et al. 1998).