TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3307 SUBJECT: GRB 050421: Swift-UVOT observation DATE: 05/04/21 17:36:02 GMT FROM: Alexander Blustin at MSSL-UCL A. Blustin (MSSL), L. Barbier (GSFC), K. McGowan (MSSL), P. Roming (PSU), M. De Pasquale (MSSL), S. T. Holland (GSFC), N. Gehrels (GSFC), K. Mason (MSSL), J. Nousek (PSU), on behalf of the Swift UVOT team Starting with the data taken ~245s after the BAT trigger (Barbier et al., GCN 3296) we have co-added the early time data of GRB 050421 in each of the Ultra-Violet/Optical Telescope (UVOT) V, B and U bands. The co-added images are made up of 9 x 10s exposures. No source is detected within the revised XRT error circle (Godet et al., GCN 3301) in any of the bands; we estimate the five sigma magnitude upper limits from the co-added images to be: Filter Upper limit V 17.5 B 19.1 U 18.8 The magnitudes are based on preliminary zero-points, measured in orbit, and will require refinement with further calibration. This message may be cited.