TITLE: GCN CIRCULAR NUMBER: 20919 SUBJECT: GRB 170318B: Swift/UVOT Upper Limits DATE: 17/03/19 08:56:36 GMT FROM: Alice Breeveld at MSSL-UCL A. A. Breeveld (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 170318B 84 s after the BAT trigger (Cholden-Brown et al., GCN Circ. 20908). No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 20910) 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 84 234 147 >20.4 u_FC 296 546 246 >19.8 white 84 6145 530 >21.0 v 626 6557 432 >19.4 b 552 5940 236 >20.0 u 296 7172 659 >20.2 w1 675 6967 432 >19.9 m2 5123 5323 197 >19.3 w2 601 6352 216 >19.6 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 4.51 in the direction of the burst (Schlegel et al. 1998).