TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3960 SUBJECT: GRB050908: detection of Swift/UVOT optical counterpart DATE: 05/09/10 21:09:41 GMT FROM: Alexander Blustin at MSSL-UCL M. De Pasquale (UCL-MSSL), M. Goad (U. Leicester), A. J. Blustin (UCL-MSSL), M. Chester (PSU), L. Angelini (GSFC-JHU), N. Gehrels (GSFC) on behalf of the Swift/UVOT team Further analysis of the early Swift/UVOT data for GRB050908 (Goad et al., GCN 3942) reveals a faint, decaying source at the position of the optical counterpart reported by Torii (GCN 3943). In summed V-band images taken between 104-650 s after the trigger, a source is visible at 19.3 +/0.1 mag in a total exposure time of 149 s. It is not detected in data taken later than this; in summed V-band images from 700-40650 s post-trigger a 3-sigma upper limit of 20.5 mag is obtained at the same position in 2296 s of co-added exposure time. The source is only detected in the V band. These magnitudes are uncorrected for extinction and are based on preliminary zero-points, measured in orbit, and will require refinement with further calibration.