TITLE: GCN CIRCULAR NUMBER: 19997 SUBJECT: GRB 161004B: Swift/UVOT Upper Limits DATE: 16/10/05 16:23:52 GMT FROM: Frank Marshall at GSFC F. E. Marshall (NASA/GSFC) and A. D'Ai (INAF-IASFPA) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 161004B 82 s after the BAT trigger (D'Ai et al., GCN Circ. 19987). No optical afterglow consistent with the XRT position (Evans et al., GCN Circ. 19991) 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 82 232 147 >20.8 u_FC 294 544 246 >19.6 white 82 1886 431 >21.2 v 624 1937 156 >19.5 b 550 1862 136 >19.6 u 294 1836 363 >19.6 w1 674 1812 136 >18.9 m2 5622 5747 123 >18.8 w2 600 1736 136 >18.7 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.54 in the direction of the burst (Schlegel et al. 1998).