[vsnet-grb-info 1996] GRB 050820 Swift-BAT refined analysis

[GCN Circulars]

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  <jayc at milkyway.gsfc.nasa.gov>

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.