TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3791 SUBJECT: GRB050813: SWIFT/UVOT observation DATE: 05/08/13 13:53:23 GMT FROM: Alexander Blustin at MSSL-UCL A. J. Blustin (UCL-MSSL), A. Retter (PSU), M. J. Page (UCL-MSSL), J. Nousek (PSU), W. Voges (MPE), N. Gehrels (GSFC) on behalf of the Swift UVOT team Swift/UVOT began observing the field of GRB050813 at 06:46:52.0 UT, 102 s after the BAT trigger (Retter et al., GCN 3788). We detect no new source with respect to the DSS within the XRT error circle (Morris et al., GCN 3790) down to the following 3-sigma upper limits: Filter T_range (s) Exp (s) 3sig UL (mag) V 103-963 188 19.1 B 249-1200 188 20.2 U 235-1096 188 19.9 UVW1 221-991 98 19.7 UVM2 207-977 98 19.3 UVW2 264-1305 188 20.6 Where T_range is the time in seconds since the BAT trigger. There is a known star at the edge of the XRT error circle at the J2000 coordinates 16:07:57.41 +11:14:42.9 with a V magnitude of 15.9 measured by the UVOT. These magnitudes are based on preliminary zero-points, measured in orbit, and will require refinement with further calibration.