TITLE: GCN CIRCULAR NUMBER: 21049 SUBJECT: GRB 170428A: Swift/UVOT Upper Limits DATE: 17/04/29 00:40:35 GMT FROM: Paul Kuin at MSSL N. P. M. Kuin (UCL-MSSL) and A. P. Beardmore (U. Leicester) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 170428A 740 s after the BAT trigger (Beardmore et al., GCN Circ. 21042). No optical afterglow consistent with the XRT position (Beardmore et al., GCN Circ. 21047) 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 initial exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag v 740 16576 1101 >20.0 b 838 10823 1101 >21.0 u 813 5412 299 >19.5 w1 789 5207 216 >19.5 m2 764 17481 1102 >20.1 w2 4392 11731 1082 >20.0 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.06 in the direction of the burst (Schlegel et al. 1998).