TITLE: GCN CIRCULAR NUMBER: 19984 SUBJECT: GRB 161004A: Swift/UVOT Detection DATE: 16/10/04 21:15:33 GMT FROM: Alice Breeveld at MSSL-UCL A. A. Breeveld (UCL-MSSL) and P. A. Evans (U. Leicester) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 161004A 85 s after the BAT trigger (Evans et al., GCN Circ. 19979). A source consistent with the enhanced XRT position (Beardmore et al., GCN Circ. 19983) is detected in the initial UVOT exposures in the white and u filters only. The preliminary UVOT position is: RA (J2000) = 17:32:36.54 = 263.15224 (deg.) Dec (J2000) = -00:57:02.7 = -0.95075 (deg.) with an estimated uncertainty of 0.56 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 85 235 147 19.47 ± 0.16 v 627 6121 432 >20.1 b 552 10926 1144 >21.8 u 297 547 246 19.55 ± 0.29 w1 848 6531 413 >20.2 m2 4690 6326 393 >20.3 w2 602 12506 1063 >20.8 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.25 in the direction of the burst (Schlegel et al. 1998).