TITLE: GCN CIRCULAR NUMBER: 21454 SUBJECT: GRB 170810A: Swift/UVOT Detection DATE: 17/08/11 04:04:43 GMT FROM: Sam LaPorte at PSU GRB 170810A: Swift/UVOT Detection S. J. LaPorte (PSU) and S. L. Gibson (U. Leicester) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 170810A 85 s after the BAT trigger (Gibson et al., GCN Circ. 21450). A source consistent with the XRT position (Goad et al. GCN Circ. 21451) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 12:31:45.33 = 187.93889 (deg.) Dec (J2000) = +03:39:38.9 = 3.66081 (deg.) with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence). Preliminary detections 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 85 235 147 14.64 +/- 0.02 v 628 648 20 15.88 +/- 0.13 b 554 574 20 16.62 +/- 0.10 u 299 1493 281 >19.3 w1 677 1473 97 >18.9 w2 604 623 19 >18.0 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.03 in the direction of the burst (Schlegel et al. 1998).