TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3330 SUBJECT: Swift Detection of the Bright Burst GRB050502b DATE: 05/05/02 10:42:36 GMT FROM: David Burrows at PSU/Swift A. Falcone, D. N. Burrows, M. Chester (PSU), P. Schady (MSSL), J. Cummings (GSFC/NRC), D. Palmer (LANL), C. Pagani (PSU), B. Zhang (UNLV), K. Page, M. Goad (U.Leicester), N. Gehrels (GSFC) on behalf of the Swift Team. At 09:25:40 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB050502b (trigger=116116). The BAT on-board calculated location is RA, Dec = 142.543, +17.002 (09:30:10, +17:00:07) (J2000) with an uncertainty of 3 arcmin (radius, 3-sigma, including estimated systematic uncertainty). The BAT light curve showed a 7-second long peak with complex structure from t-4 to t+3. The maximum count rate was 4000 counts per second (15-350 keV) in the 1 second time interval centered around t+1. The spacecraft slewed promptly and the XRT began observations at 09:26:43 UT. The initial centroiding image detected only 5 counts, insufficient to produce a centroid. Further analysis will require the ground-processed data, which will be available at the Swift MOC at about 12:30 UT. The Swift Ultra Violet/Optical (UVOT) observations began at 09:26:40 UT, 60 seconds after the BAT trigger. The first data taken after the spacecraft settled was a 100 sec exposure using the V filter with the midpoint of the observation at 135 sec after the BAT trigger. This image was sent directly to the ground via TDRSS with the FOV pixel binning set at 8 x 8 to reduce telemetry. A comparison against the USNO-B1 reveals a new source inside the BAT error circle at RA, Dec 09:30:12.58, +17:02:00.7 (J2000). This is 118 arcseconds from the BAT position. The V-band magnitude was 17.7 +/- 0.1.