TITLE: GCN CIRCULAR NUMBER: 20914 SUBJECT: GRB 170318A: Swift/UVOT Upper Limits DATE: 17/03/18 21:27:48 GMT FROM: Alice Breeveld at MSSL-UCL A. A. Breeveld (UCL-MSSL) and B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 170318A 117 s after the BAT trigger (Sbarufatti et al., GCN Circ. 20904). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 20907) 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 117 266 147 >20.4 u_FC 329 552 220 >19.6 white 117 6605 363 >20.8 v 2222 2241 19 >17.0 b 2147 6400 216 >19.8 u 329 6195 417 >20.1 w1 5790 5990 197 >19.4 m2 5585 5784 197 >19.3 w2 2197 11766 560 >20.4 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.97 in the direction of the burst (Schlegel et al. 1998).