TITLE: GCN CIRCULAR NUMBER: 21315 SUBJECT: GRB170710A: Swift/UVOT detection DATE: 17/07/10 19:37:22 GMT FROM: Alice Breeveld at MSSL-UCL A. A. Breeveld (UCL-MSSL), F. E. Marshall (NASA/GSFC) 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 170710A 132 s after the BAT trigger (Evans et al., GCN Circ. 21308). An optical afterglow consistent with the enhanced XRT position (Osborne et al., GCN Circ. 21310) and Gemini position (Cuchhiara, GCN Circ. 21311) is detected in the initial UVOT white finding chart. Preliminary detection 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 132 281 147 20.95 +/- 0.3 white 5808 6007 197 >21.0 v 2122 2141 19 >17.9 b 546 565 19 >18.9 u 290 539 246 >20.2 w1 2171 2166 20 >17.1 w2 2098 2117 20 >17.6 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.172 in the direction of the burst (Schlegel et al. 1998).