[vsnet-grb-info 7917] GRB 090531: Swift-BAT refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9460
SUBJECT: GRB 090531: Swift-BAT refined analysis
DATE:    09/05/31 16:36:42 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at lheamail.gsfc.nasa.gov>

T. Sakamoto (GSFC/UMBC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), E. E. Fenimore (LANL), N. Gehrels (GSFC),
H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD), D. M. Palmer (LANL),
G. Sato (ISAS), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC),
T. N. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-60 to T+243 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 090531 (trigger #353627)
(Stamatikos, et al., GCN Circ. 9454).  The BAT ground-calculated position is
RA, Dec = 178.672, 7.824 deg, which is 
   RA(J2000)  =  11h 54m 41.4s 
   Dec(J2000) = +07d 49' 27.0" 
with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 73%.
 
The mask-weighted light curve shows a single peak starting at ~T-25 sec,
peaking at T+9 sec, and ending at ~T+60 sec.  There is a possible (2.5 sigma CL)
episode of emission from T+80 to ~T+140 sec.  T90 (15-350 keV) is 32 +- 6 sec
(estimated error including systematics).
 
The time-averaged spectrum from T-12.6 to T+29.4 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 0.86 +- 0.42, 
and Epeak of 64.7 +- 14.3 keV (chi squared 51.91 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 1.3 +- 0.1 x 10^-6 erg/cm2
and the 1-sec peak flux measured from T+9.29 sec in the 15-150 keV band is
1.3 +- 0.2 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.68 +- 0.09 (chi squared 65.50 for 57 d.o.f.).  All the quoted errors
are at the 90% confidence level. 
 
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/353627/BA/