TITLE: GCN CIRCULAR NUMBER: 18462 SUBJECT: GRB 151023A: Swift/UVOT Upper Limits DATE: 15/10/24 02:15:35 GMT FROM: Mike Siegel at PSU/Swift MOC M. H. Siegel (PSU), B. G. Balzer (PSU) and P. D'Avanzo (INAF-OAB) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 151023A 796 s after the BAT trigger (D'Avanzo et al., GCN Circ. 18450). No optical afterglow consistent with the XRT position (D'Avanzo et al. GCN Circ. 18450) 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 796 946 147 >19.6 white 796 2140 411 >20.0 v 1130 2191 136 >18.5 b 1229 2116 117 >19.3 w1 1354 1374 19 >18.2 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 1.42 in the direction of the burst (Schlegel et al. 1998).