TITLE: GCN CIRCULAR NUMBER: 21411 SUBJECT: GRB 170803A: Swift/UVOT Detection DATE: 17/08/04 13:04:24 GMT FROM: Sam Emery at MSSL-UCL S.W.K. Emery (UCL-MSSL) and S. J. LaPorte (PSU) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 170803A 88 s after the BAT trigger (LaPorte et al., GCN Circ. 21405). GRB 170803A was also triggered by Fermi GBM (Veres and Meegan GCN Circ. 21406) A fading source consistent with the XRT position (Beardmore et al., GCN Circ. 21407) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 11:39:44.11 = 174.93380 (deg.) Dec (J2000) = -16:18:38.0 = -16.31055 (deg.) with an estimated uncertainty of 0.44 arc sec. (radius, 90% confidence). Preliminary 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 88 238 147 18.10 +/- 0.06 white 580 2241 322 19.69 +/- 0.12 u_FC 300 550 246 18.58 +/- 0.16 u 775 2366 175 >19.34 b 556 2386 191 20.02 +/- 0.35 uvw1 6216 23821 982 >20.74 uvm2 17025 17715 679 >20.57 uvw2 5602 30135 990 >21.05 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).