GCN Circular 3408
Derek B. Fox (Caltech) reports on behalf of a larger collaboration: "We have created a summed image from the first 10.14 ksec of exposure on GRB050416 (BAT trigger #114753; Sakamoto et al., GCN 3264) through the "UVW2" filter of the Swift UVOT. Earlier reports have discussed the detection of the optical afterglow of this event (Cenko & Fox, GCN 3265) in the U, B, and V filters (Schady et al., GCN 3280). The UVW2 sequence consists of nine ~10-second exposures taken from 4 to 15 minutes after the burst; two ~100-second exposures taken 18 and 29 minutes after the burst, and 11 ~900-second exposures taken from 92 minutes to 2 days after the burst. The final summed frame was astrometrically registered to the USNO B-1.0 catalog. At the position of the optical afterglow we detect a source at >3-sigma confidence with a mean count rate of 6.0 +/- 0.7 cts/ksec according to the default uvotdetect analysis. Examination of individual frames suggests that the source is not distinguishable above background in the individual short (10-s and 100-s) exposures, being most prominent in the early ~900 second exposures of the sequence. The UVW2 filter bandpass is centered at 188 nm with a width of 76 nm. Since it is unlikely that significant afterglow emission would be detected blueward of the host galaxy Lyman limit at 91 nm -- with significant suppression redward to the Lyman-alpha resonance at 122 nm -- we suggest that this detection implies a redshift constraint for GRB050416 of z <~ 1.0."