TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3964
SUBJECT: GRB 050911: Swift-BAT refined analysis
DATE:    05/09/11 19:24:11 GMT
FROM:    Takanori Sakamoto at NASA/GSFC  <takanori@milkyway.gsfc.nasa.gov>

J. Tueller (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), 
A. Parsons (GSFC), T. Sakamoto (GSFC/NRC), G. Sato (ISAS), 
on behalf of the Swift-BAT team:

Using the data set from T-60 to T+120 sec from the recent telemetry
downlink, we report further analysis of Swift-BAT GRB 050911
(trigger #154630)  (Page, et al., GCN 3961).  The refined BAT ground 
position is (RA,Dec) = 13.718, -38.862 { 0h 54m 52.4s, -38d 51' 42.8"} 
[deg; J2000] +-2.8 arcmin, (radius, sys+stat, 90% containment).  
The partial coding was 43 %.  

The mask-weighted BAT light curves show at least three peaks.  
The first two peaks are at T-1 and T+0 sec, each with a duration of 
0.5 sec. The third peak is a slow rise and decay from T+10 to T+20 sec.   
The third peak is not appreciably softer than the first two, 
as suggested by Berger (GCN Circ 3962).  Thus the burst is very likely 
in the long GRB category.  T90 (15-350 keV) is (16 +- 2) sec.  
(estimated error including systematics).  

The power law index of the time-averaged spectrum is 1.87 +- 0.03.  
The fluence in the 15-150 keV band is (3.0 +- 0.6) x 10^-7 erg/cm2.  
The 1-sec peak photon flux measured from T0-0.1 sec in the 15-150 keV 
band is (1.3 +- 0.2) ph/cm2/sec.  All the quoted errors are at the 90% 
confidence level.