TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3339 SUBJECT: GRB050502B Swift-BAT refined analysis DATE: 05/05/03 01:15:39 GMT FROM: Jay R. Cummings at NASA/GSFC/Swift J. Cummings (GSFC/NRC), L. Barbier (GSFC), S. Barthelmy (GSFC), E. Fenimore (LANL), N. Gehrels (GSFC), D. Hullinger (UMD), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (GSFC/NRC), G. Sato (ISAS), M. Suzuki (Saitama), M. Tashiro (Saitama U.), J. Tueller (GSFC), on behalf of the Swift/BAT team: At 09:25:40 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB050502B (trigger=116116) (GCN Circ 3330, Falcone et al.). The refined BAT ground position is (RA,Dec) = 142.543, +17.002, [deg; J2000] +- 3 arcmin, (95% containment). This is 24 arc seconds from the position determined by the XRT (GCN Circ 3333, Pagani, et al.). The burst was 10 degrees off the Swift boresight, within the fully-coded BAT field of view. The BAT mask-weighted light curve shows one main peak of about 6 seconds duration centered at the trigger time preceded by two smaller peaks 16 and 10 seconds earlier with a softer spectrum. T90 (15-350 keV) is (17.5 +- 0.2) seconds (estimated error including systematics). The spectrum is well fit by a single power law with photon index of 1.6 +- 0.1. The fluence in the 15-350 keV band is (8.0 +- 1.0) x 10^-7 erg/cm2. The peak photon flux in the 15-350 band is (1.73 +- 0.15) ph/cm2/s. All the quoted errors are at the 90% confidence level. We note that this burst appears to be well-suited for ground follow-up observations, as it is located 90 degrees from the Sun, 180 degrees from the Moon, close to the ecliptic, and away from the Galactic center.