TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3757
SUBJECT: GRB 050803: Swift-BAT refined analysis 
DATE:    05/08/04 15:06:33 GMT
FROM:    Craig Markwardt at NASA/GSFC/UMD  <craigm@milkyway.gsfc.nasa.gov>

A. Parsons (GSFC), L. Barbier (GSFC), S. Barthelmy (GSFC),  
M. Chester (PSU), J. Cummings (GSFC/NRC), E. Fenimore (LANL),  
N. Gehrels (GSFC), D. Hullinger (UMD), H. Krimm (GSFC/USRA),  
C. Markwardt (GSFC/UMD), F. Marshall (GSFC), D. Palmer (LANL),  
T. Sakamoto (GSFC/NRC), G. Sato (ISAS), J. Tueller (GSFC),  
on behalf of the Swift-BAT team: 

 
At 19:14:00 UT Swift-BAT detected GRB 050803 (trigger=148833) (GCN 
Circ 3748, Band, et al.).  The refined BAT ground position is (RA,Dec) 
= 350.658, +5.778 {23h22m37.9s, +05d46'40.8"} [deg; J2000] +-3 arcmin, 
(95% containment).  The partial coding was 46%. 
 
The light curve has one broad and one narrow peak, of approximately 
equal peak count rates.  The broad peak extends from T+60 to T+100 
sec, with a roughly symmetrical shape.  The narrow peak is centered at 
T+148 sec, with a duration of ~2 sec.  There is some inter-peak 
emission and some emission after the narrow peak for ~20 sec.  
The formal T90 is 85 +- 10 sec, and T50 is 
30 +- 5 sec (15-350 keV; estimated error including systematics). 
However, due to the large spread between the two peaks, a more 
accurate measure of the total burst duration is ~110 sec. 
 
We note this burst was the result of a 72-sec long BAT Image Trigger 
whose integration started ~55 sec before the onset of the broad peak 
(i.e. the integration included ~15 sec of that first peak).  Hence, 
the actual start of the burst ocurred 19:14:55 UT.  

A simple power law fit to the time averaged spectrum produces a photon 
index of 1.5 +- 0.1.  The fluence is (3.9 +- 0.3) x 10^-6 erg/cm2. 
The 1-s peak photon flux measured from T+147 sec is (1.5 +- 0.2) 
ph/cm2/s (this corresponds to the narrow peak).   
 
The narrow peak contains ~5% of the total fluence.  Its photon index 
is (1.4 +-0.2), consistent with the overall average spectrum. 
 
All fluxes and fluences are quoted for the 15-350 keV band, and the 
quoted errors are at the 90% confidence level.