TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3129 SUBJECT: GRB 050319: Swift-UVOT detection of the afterglow DATE: 05/03/21 00:38:06 GMT FROM: Martin Still at NASA/GSFC Swift SSC P. Boyd (GSFC/UMBC), S. T. Holland, M. Still (GSFC/USRA), W. Landsman (GSFC), S. Hunsberger (PSU), A. Breeveld (MSSL), P. Roming (PSU), K. Mason, P. Schady (MSSL), M. Ivanushkina (PSU), T. Poole (MSSL), C. Gronwall (PSU) A. Blustin (MSSL), P. Brown (PSU), S. Rosen, K. McGowan (MSSL), S. Koch (PSU), M. Carter, H. Huckle, M. De Pasquale (MSSL), P. Broos (PSU), T. Kennedy, P. Smith, B. Hancock (MSSL), H. Krimm (GSFC/USRA), J. Nousek (PSU), N. Gehrels (GSFC), on behalf of the Swift UVOT team. The Swift Ultra-Violet/Optical Telescope (UVOT) began settled observations of the field of GRB 050319 (Krimm et al; GCN 3117) at 09:32:48 UT, 90 after the BAT trigger. A comparison of the 100s finding chart (with mid-exposure 140s after the trigger) against the Digitized Sky Survey reveals a new source inside the BAT and XRT error circles at: RA = 10h 16m 47.76(3)s Dec = +43d 32m 54.9(5)s J2000 and consistent with the ROTSE-IIIb position reported by Rykoff et al. (GCN 3116) The V band magnitude is V = 17.5(1). The UVOT obtained a series of images of this burst with a variety of integration times. Mid-exposure times, Delta-T, with respect to the BAT trigger at 09:31:18 UT are measured in seconds. Observations with the V and B filters are listed below: Delta-T Exposure(s) Filter Mag 140 100 V 17.5 (Finding Chart) 380 80 B 18.8 600 80 V 18.4 1030 100 B 18.9 1250 100 V 19.2 5400 900 B 20.9 7080 700 V 20.2 17000 900 B 21.2 18700 700 V 20.6 No signal was detected in the U band in a total exposure time of 90s centered approximately 600s after the trigger with an upper limit of U = 19. We caution that the absolute magnitude scale is based on ground calibration and will require confirmation in flight.