TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3835 SUBJECT: GRB 050820 Swift-BAT refined analysis DATE: 05/08/20 14:11:03 GMT FROM: Jay R. Cummings at NASA/GSFC/Swift J. Cummings (GSFC/NRC), L. Barbier (GSFC), S. Barthelmy (GSFC), J. Cannizzo (GSFC-UMBC), L. Cominsky (Sonoma State U.), D. Hullinger (GSFC/UMD), E. Fenimore (LANL), N. Gehrels (GSFC), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (GSFC/NRC), G. Sato (ISAS), J. Tueller (GSFC) on behalf of the Swift-BAT team: Using the full data set from the recent telemetry downlink, we report further analysis of Swift-BAT GRB 050820 (trigger #151207) (Page, et al., GCN 3830). The ground-analysis position is RA,Dec 337.418, +19.560 (J2000) with an uncertainty of 2 arcmin (radius, 90%, stat+sys). This is 31 arcseconds from the optical transient reported by Fox et al. in GCN Circ. 3829. The light curve is multi-peaked and the spectrum clearly evolves from hard to soft within each of the two main peaks. There is a probable small precursor at ~ T-15 seconds, the largest peak at T+0 seconds, and two other peaks at T+9 and T+13 seconds. T90 is 26 +- 2 seconds. Fitting a simple power law over the interval from T-17 to T+22 seconds, the photon index is 1.7 +/- 0.1 with a fluence of 1.9 +/- 0.2 x 10^-6 erg/cm^2 in the 15-350 keV band (90% c.l). The peak flux in a 1-second wide window starting at T-0.25 seconds is 1.3 +/- 0.2 ph/cm^2/sec (15-350 keV). The isotropic-equivalent energy using the redshift of 2.612 (Prochaska, et al. GCN 3833) is 9.7 (-2.6/+3.5) x 10^51 ergs in the 4.2-41.5 keV band in the GRB rest frame (15-150 keV band in the observer's frame) using a simple power-law model.