TITLE: GCN CIRCULAR NUMBER: 21220 SUBJECT: GRB 170607A: Swift/UVOT Detection DATE: 17/06/08 12:56:36 GMT FROM: Sam Emery at MSSL-UCL S.W.K. Emery (UCL-MSSL) and A. D'Ai (INAF-IASFPA) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 170607A 84 s after the BAT trigger (D'Ai et al., GCN Circ. 21214). GRB 170607A was also triggered by Fermi GBM (Hamburg and Meegan GCN Circ. 21218) We detect a fading afterglow consistent with the position given by XRT (Beardmore et al., GCN Circ. 21215) and ground-based optical detections (Heintz et al., GCN Circ. 21217; Lipunov et al., GCN Circ. 21219). The preliminary UVOT position is: RA (J2000) = 00:29:27.82 = 7.36591 (deg.) Dec (J2000) = +09:14:36.0 = 9.24334 (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 first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white_FC 84 233 147 15.76 +/- 0.02 white 575 5285 461 17.11 +/- 0.03 white 6895 7065 197 17.50 +/- 0.04 v 625 5696 313 17.51 +/- 0.11 b 551 1859 132 17.63 +/- 0.09 u_FC 295 545 246 16.14 +/- 0.04 u 699 6205 210 16.80 +/- 0.06 u 6454 29372 1601 17.68 +/- 0.04 uvw1 847 6105 313 16.87 +/- 0.06 uvm2 5700 5900 197 16.86 +/- 0.09 uvw2 1720 5491 216 17.04 +/- 0.07 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).