TITLE: GCN CIRCULAR NUMBER: 20074 SUBJECT: GRB 161017A: Swift/UVOT Detection DATE: 16/10/18 12:34:01 GMT FROM: Alice Breeveld at MSSL-UCL A. A. Breeveld (UCL-MSSL) and E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 161017A 68 s after the BAT trigger (Troja et al., GCN Circ. 20064, Yurkov et al., GCN Circ. 20063). The UVOT source described in Troja et al., GCN Circ. 20064, is at a position consistent with the enhanced XRT position (Beardmore et al., GCN Circ. 20067) and is found to be fading. The preliminary UVOT position is: RA (J2000) = 09:31:04.60 = 142.76915 (deg.) Dec (J2000) = +43:07:36.3 = 43.12676 (deg.) with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence). Preliminary detections 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 68 217 147 14.08 ± 0.02 v 609 629 20 15.14 ± 0.09 b 535 555 20 15.60 ± 0.06 u 280 529 246 14.56 ± 0.02 uvw1 658 661 2.7 15.5 ± 0.3 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.02 in the direction of the burst (Schlegel et al. 1998).