TITLE: GCN CIRCULAR NUMBER: 16710 SUBJECT: GRB 140817A: Swift/UVOT Afterglow Detection DATE: 14/08/18 21:40:21 GMT FROM: Frank Marshall at GSFC F. E. Marshall (NASA/GSFC) and B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 140817A 101 s after the BAT trigger (Sbarufatti et al., GCN Circ. 16695). A fading source consistent with the UVOT-enhanced XRT position (Osborne et al. GCN Circ. 16697) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 08:29:03.21 = 127.26336 (deg.) Dec (J2000) = 58:11:24.7 = 58.19020 (deg.) with an estimated uncertainty of 0.53 arc sec. (radius, 90% confidence). Preliminary detection 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 101 1017 334 20.3 +/- 0.17 white 5426 18414 1794 >21.6 v 642 1068 58 >18.0 b 568 762 39 >18.5 u 313 4899 435 >20.5 w1 692 1117 39 >19.2 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.15 in the direction of the burst (Schlegel et al. 1998).