TITLE: GCN CIRCULAR NUMBER: 19764 SUBJECT: GRB 160804A: Swift/UVOT Detection DATE: 16/08/04 11:40:54 GMT FROM: Alice Breeveld at MSSL-UCL A. A. Breeveld (UCL-MSSL) and F. E. Marshall (NASA/GSFC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 160804A 82 s after the BAT trigger (Marshall et al., GCN Circ. 19761). A fading source consistent with the XRT position (Osborne et al. GCN Circ. 19762) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 14:46:31.14 = 221.62976 (deg.) Dec (J2000) = +09:59:56.7 = 9.99908 (deg.) with an estimated uncertainty of 0.44 arc sec. (radius, 90% confidence). Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white 82 232 147 18.56 ± 0.06 white 794 944 148 19.66 ± 0.12 v 570 6762 373 >19.9 b 496 6227 432 20.4 ± 0.24 u 240 490 246 19.53 ± 0.20 w1 4182 4382 197 18.90 ± 0.25 m2 1000 5612 216 >18.7 w2 546 6637 432 >19.5 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.03 in the direction of the burst (Schlegel et al. 1998).