TITLE: GCN CIRCULAR NUMBER: 21386 SUBJECT: GRB 170728A: Swift/UVOT Detection DATE: 17/07/29 21:09:59 GMT FROM: Sam LaPorte at PSU GRB 170728A: Swift/UVOT Detection S. J. Laporte (PSU) and J. K. Cannizzo (NASA/UMBC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 170728A 74 s after the BAT trigger (Cannizzo et al., GCN Circ. 21367). A fading source consistent with the XRT position (Goad et al. GCN Circ. 21368) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 03:55:33.17 = 58.88819 (deg.) Dec (J2000) = +12:10:54.7 = 12.18187 (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 70 219 147 19.20+-0.17 white 70 4048 342 19.93+-0.18 v 4264 5808 267 >19.1 b 3655 11916 345 >19.7 u 287 22167 1283 >20.0 w1 6020 21852 1105 >20.1 m2 5814 16112 1082 >20.6 w2 4059 4259 197 >19.6 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.24 in the direction of the burst (Schlegel et al. 1998).