TITLE: GCN CIRCULAR NUMBER: 20851 SUBJECT: GRB 170311A: Swift/UVOT Upper Limits DATE: 17/03/12 02:10:46 GMT FROM: Mike Siegel at PSU/Swift MOC M. H. Siegel (PSU) and J. R. Cummings (NASA/UMBC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 170311A 110 s after the BAT trigger (Cummings et al., GCN Circ. 20846). No optical afterglow consistent with the XRT position (Cummings et al. GCN Circ. 20846) 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 110 259 147 >20.5 u_FC 269 519 246 >19.7 white 110 7074 645 >21.2 v 600 11424 1022 >20.3 b 525 5585 294 >20.1 u 269 5380 537 >20.1 w1 650 5175 279 >20.1 m2 624 13058 82 >20.1 w2 575 7280 313 >20.2 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.16 in the direction of the burst (Schlegel et al. 1998).