TITLE: GCN CIRCULAR NUMBER: 21207 SUBJECT: GRB 170604A: Swift/UVOT Observations DATE: 17/06/05 19:10:10 GMT FROM: Mike Siegel at PSU/Swift MOC M. H. Siegel (PSU) and K. L. Page (U. Leicester) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 170604A 128 s after the BAT trigger (Page et al., GCN Circ. 21191). We detect a fading afterglow consistent with the position given by XRT (Page et al., GCN Circ 21196) and ground-based optical detections (Izzy et al., GCN Circ 21197; Schady, GCN Circ. 21200; Mazaeva et al., GCN Circ 21203). The preliminary UVOT position is: RA (J2000) = 22:50:37.52 = 342.65632 (deg.) Dec (J2000) = -15:24:43.9 = -15.41220 (deg.) with an estimated uncertainty of 0.1 arc sec. (radius, 90% confidence). Preliminary magnitudes and 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) 128 278 147 15.81+-0.03 white 3968 5603 393 18.72+-0.06 white 39053 39959 885 20.57+-0.18 v 4378 6013 393 18.94+-0.31 v 34480 44756 1268 >19.89 b 3763 5398 393 19.26+-0.15 b 38141 39048 885 >20.86 u (fc) 287 429 279 15.70+-0.03 u 4993 5193 196 18.51+-0.16 u 28543 29132 576 >19.72 uvw1 4788 4988 196 18.47+-0.19 uvw1 23318 46354 1633 20.20+-0.27 uvm2 4583 6169 344 18.58+-0.18 uvm2 44762 45661 885 >20.37 uvw2 4174 5809 393 19.53+-0.26 uvw2 32392 40613 1700 >20.86 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.04 in the direction of the burst (Schlegel et al. 1998).