TITLE:   GCN CIRCULAR
NUMBER:  9939
SUBJECT: GRB 090926B: Swift-BAT refined analysis
DATE:    09/09/27 03:51:16 GMT
FROM:    Jay R. Cummings at NASA/GSFC/Swift  <james.r.cummings@nasa.gov>

W. H. Baumgartner (GSFC/UMBC), S. D. Barthelmy (GSFC),
J. R. Cummings (GSFC/UMBC), D. Grupe (PSU), E. E. Fenimore (LANL),
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD),
D. M. Palmer (LANL), A. M. Parsons (GSFC), T. Sakamoto (GSFC/UMBC),
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-61 to T+243 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 090926 (trigger #370791)
(Grupe, et al., GCN Circ. 9935).  The BAT ground-calculated position is
RA, Dec = 46.310, -38.997 deg which is
    RA(J2000)  =  03h 05m 14.3s
    Dec(J2000) = -38d 59' 50.9"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 26%.

The mask-weighted light curve shows a sharp rise at T-20 sec, then a
complex peak reaching a maximum at T-1 sec, followed by an approximately
exponential decay. T90 (15-350 keV) is 109.7 +- 11.3 sec (estimated 
error including systematics).

The time-averaged spectrum from T-21.6 to T+133.2 sec is best fit by a
power law with an exponential cutoff.  This fit gives a photon index
0.52 +- 0.24, and Epeak of 78.3 +- 7.0 keV (chi squared 57.75 for 56
d.o.f.).  For this model the total fluence in the 15-150 keV band is
7.3 +- 0.2 x 10^-06 erg/cm2 and the 1-sec peak flux measured from
T-1.31 sec in the 15-150 keV band is 3.2 +- 0.3 ph/cm2/sec.  A fit to a
simple power law gives a photon index of 1.54 +- 0.00 (chi squared
125.34 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/370791/BA/