[vsnet-grb-info 5341] GRB 071031, Swift-BAT refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  7029
SUBJECT: GRB 071031, Swift-BAT refined analysis
DATE:    07/10/31 20:26:43 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at lheamail.gsfc.nasa.gov>

M. Stamatikos (GSFC/ORAU), S. D. Barthelmy (GSFC), J. Cummings (GSFC/UMBC),
E. Fenimore (LANL), N. Gehrels (GSFC), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), D. Palmer (LANL), T. Sakamoto (GSFC/UMBC),
G. Sato (GSFC/ISAS), M. C. Stroh (PSU), J. Tueller (GSFC),
T. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-119 to T+297 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 071031 (trigger #295670)
(Stroh, et al., GCN Circ. 7020).  The BAT ground-calculated position is
RA, Dec = 6.399, -58.048 deg, which is 
   RA(J2000)  =  00h 25m 35.8s 
   Dec(J2000) = -58d 2' 51" 
with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 75%.
 
The mask-weighted light curve shows multiple peaks.  The first starts
at ~T-10 sec, peaks at ~T+5 sec, and is essentially back to baseline
by ~T+40 sec. The next largest peak starts at ~T+100 sec, peaks
at ~T+105 sec, and returns to baseline by ~T+180 sec.
T90 (15-350 keV) is 180 +- 10 sec (estimated error including systematics).
 
The time-averaged spectrum from T-4.5 to T+192.5 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.42 +- 0.29.  The fluence in the 15-150 keV band is 9.0 +- 1.3 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+2.92 sec in the 15-150 keV band
is 0.5 +- 0.1 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level. 
 
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 1.34.  This fluence ratio is larger
than 1.32 which can be achieved in the 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.