GCN Circular 8338

GRB 081007, Swift-BAT refined analysis

C. M. Markwardt (UMD/GSFC),
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),
K. McLean (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-120 to T+182 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 081007 (trigger
#330856) (Baumgartner et al., GCN Circ. 8330
Not found
 
 
).  The BAT
ground-calculated position is RA, Dec = 339.963, -40.146 deg which is
     RA(J2000)  =  22h 39m 51.1s
     Dec(J2000) = -40d 08' 44.8"
with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90%
containment).  The partial coding was 28%.

The mask-weighted light curve shows a large peak of approximately 9
seconds duration at T+0.  This peak is seen mostly below 50 keV; a
cutoff-powerlaw model fit yields an Epeak of approximately 12 keV.
T90 (15-350 keV) is 10.0 +- 4.5 sec (estimated error including
systematics).

The time-averaged spectrum from T-6.9 to T+7.1 sec is best fit by a
simple power-law model.  The power law index of the time-averaged
spectrum is 2.51 +- 0.20.  The fluence in the 15-150 keV band is 7.1
+- 0.8 x 10^-07 erg/cm2.  The 1-sec peak photon flux measured from
T+1.10 sec in the 15-150 keV band is 2.6 +- 0.4 ph/cm2/sec.  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/330856/BA/

We note that the fluence ratio in a simple power-law fit between the
25-50 keV band and the 50-100 keV band is 1.43.  This fluence ratio is
larger than 1.32 which can be achieved with a Band function of
alpha=-1.0, beta=-2.5, and Epeak=30 keV.  Thus, preliminary analysis
shows that Epeak of the burst is very likely around or below 30 keV.
Therefore the burst can be classified as an X-ray flash (e.g. Sakamoto
et al. ApJ, 679, 570).