TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3748 SUBJECT: GRB050803: Swift-BAT detection of a very long burst DATE: 05/08/03 20:10:23 GMT FROM: Scott Barthelmy at NASA/GSFC D.L. Band (GSFC-UMBC), S. Barthelmy (GSFC), P. Boyd (GSFC-UMBC), P. Brown (PSU), D. Burrows (PSU), M. Chester (PSU), J. Cummings (GSFC/NRC), N. Gehrels (GSFC), C. Markwardt (GSFC), J. Nousek (PSU), K. Page (U. Leicester), C. Pagani (PSU), D. Palmer (LANL), T. Sakamoto (GSFC), J. Racusin (PSU) on behalf of the Swift team: At 19:14:00 UT, Swift-BAT triggered and located GRB050803 (trigger=148833). The spacecraft slewed immediately. The BAT on-board calculated location is RA,Dec 350.650d,+5.800d {23h 22m 36s,+05d 48' 00"} (J2000), with an uncertainty of 3 arcmin (radius, 90% containment, stat+sys). The BAT light curve shows a multi-peak structure with a total duration of at least 150 sec. There is a fairly strong short peak at T+145 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~80 seconds after the trigger. The XRT began observing at 19:16:31.6 UT, 151.6 seconds after the BAT trigger. An uncatalogued, rapidly fading X-ray source was detected at an RA and Dec of 23h 22m 37.7s, 05d 47m 7.4s (J2000; 90% containment uncertainty of 6 arcsec), which is 58.4" from the BAT position. The UVOT began settled exposures about 3 minutes after the burst. Using a preliminary ground-processed 100-sec image, the upper limit is 18.5 (3 sigma) in the V band. We are currently in the portion of the orbits where the spacecraft does not pass over the Malindi downlink station. Therefore, it will be ~4 hours before we have access to the full data set for the refined analyses.