TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3954 SUBJECT: GRB050908: Swift/UVOT Early upper limits DATE: 05/09/08 18:51:53 GMT FROM: Massimiliano de Pasquale at MSSL-UCL M. De Pasquale (MSSL), M. Goad (U. Leicester), F. Marshall (GSFC), J. Norris(GSFC), K. Hurley (Berkeley), N. Gehrels, on behalf of the Swift/UVOT team The Swift/UVOT began observing the field of GRB050908 at 05:44:15 UT, 104 seconds after the BAT trigger (Goad et al., GCN 3942). No new source with respect to the DSS was detected in the initial 100 s V-band UVOT image with a significance of 3 sigma at the position of the optical afterglow found by Torii et al. (GCN 3943). The corresponding 3 sigma upper limit is 18.8. There was also no optical/UV counterpart detected in the summed images from all filters, down to the following 3 sigma magnitude upper limits: Filter T_range(s) Exp (s) 3sig UL V 104-6922 942 19.9 B 250-5254 1020 21.1 U 236-1098 190 19.5 UVW1 223-993 100 19.1 UVM2 208-11040 987 20.9 UVW2 265-6163 997 20.6 Where T_range is the time post-trigger over which the summed images were accumulated and Exp is the total exposure time. The magnitudes upper limits are not corrected for extinction. These magnitudes are based on preliminary zero- points, measured in orbit, and will require refinements with further calibrations.