TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3858
SUBJECT: GRB 050820A BAT observations of second, larger episode of emission
DATE:    05/08/22 19:55:41 GMT
FROM:    Jay R. Cummings at NASA/GSFC/Swift  <jayc@milkyway.gsfc.nasa.gov>

J. Cummings (GSFC/NRC), L. Barbier (GSFC), S. Barthelmy (GSFC),
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:

A second, larger, harder episode of gamma rays from T+217 to T+270
sec was detected from GRB 050820A, as noted by Golenetskii et al. 
in GCN circ. 3846, 3852.  We confirm that this larger episode of 
emission was from the same source as the first episode (from T-17
to T+22 sec), using BAT mask-weighting on a partial data set. 

Swift was entering the SAA as the first episode of emission was 
ending.  We reported BAT refined analysis for the period covering 
the first episode of emission in GCN circ. 3835, 3848.  The BAT 
entered SAA mode at T+241 sec.  At this time data collection became 
very limited.  We use mask-weighted data prior to this time, and a 
non-directional residual rate with a background model subtraction 
after this time, for the following information on the second 
episode:

There were two peaks at T+228 and T+259 sec.  T90 was 50 +/- 5 sec 
(for the second episode alone).  In a simple power law fit, the 
photon index for the first part of the second episode from T+217 
to T+241 was 1.06 +/- 0.03.  The 1-sec peak flux is estimated to 
be 6 +/- 1 ph/cm^2/sec (15-350 keV) at T+259 sec.  For reference, 
the previously reported photon index for the first episode of 
emission from T-17 to T+22 sec was 1.7 +/- 0.1 and the 1-sec peak 
flux was 1.3 +/- 0.2 ph/cm^2/sec (15-350 keV).  

We estimate the total fluence for the entire event including both 
episodes to be 8.4 +/- 0.3 x 10^-6 erg/cm^2 in the 15-150 keV band.  
The quoted uncertainty is statistical. Roughly 60% of the fluence 
is in the period lacking mask-weighted data and having a very high
background rate, thus systematic errors may be large and we 
estimate them to be +/- 0.6 x 10^-6 erg/cm^2.  The fluence estimate 
includes the assumption that the part of the second episode after 
T+241 sec had the same spectrum as it did from T+217 to T+241 sec.

For the entire burst then, using the measured redshift of 2.612 
(Prochaska, et al. GCN 3833), we calculate the isotropic-equivalent 
energy in the 54-542 keV band in the GRB rest frame (15-150 keV 
band in the observer's frame) as ~1.5 x 10^53 ergs.
-- 
Jay Cummings