TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3923 SUBJECT: GRB 050904: Swift/UVOT observations DATE: 05/09/05 17:50:27 GMT FROM: Antonino Cucchiara at PSU A. Cucchiara (PSU), J. Cummings (GSFC), S. Holland (GSFC), C. Gronwall (PSU), A. Blustin (MSSL), F. Marshall (GCFS), A. Smale (NASA HQ), L. Cominsky (Sonoma State U.), N. Gehrels (GSFC) on behalf of the Swift UVOT team Swift/UVOT began observing the field of GRB050904 at 01:54:25 UT, 164 s after the BAT trigger (Cummings, et al., GCN 3910) in all the six filters. In the first 100 s V-band image (T+ 214 s), no new source is detected with respect to the DSS down to a 3 sigma upper limit of 18.9 mag in the XRT error circle (GCN 3910). At T+90 min after the trigger, in a single 900 s V-band image the upper limit was 20.1. The instrument continued to collect data during all the other orbits detecting no new source down to the following upper limits: Filter Start_Time(UT) Stop_Time(UT) Exp(s) 3sig_limit V 01:54 16:20 4887 20.9 B 01:56 18:12 4773 21.9 U 01:56 17:57 4686 21.6 UVW1 01:56 16:49 4292 21.1 UVM2 01:56 16:35 4684 21.4 UVW2 01:57 18:26 4331 21.4 Where Start_Time and Stop_Time are the time range over wich the summed images were accumulated and Exp is the total exposure time (in seconds) of the summed image. The magnitude upper limits are not corrected for extinction. These non-detections are consistent with a high redshift GRB, as reported by J. Haislip, et al. (GCN 3914 and 3919). These magnitudes are based on preliminary zero-points, measured in orbit, and will require refinement with further calibration. This message can be cited.