GCN Circular 15508

A. Y. Lien (NASA/UMBC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
C. B. Markwardt (GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU), M. Stamatikos (OSU),
J. Tueller (GSFC), T. N. Ukwatta (MSU)
(i.e. the Swift-BAT team):

Using the data set from T-250 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 131120A (trigger #578227)
(Maselli, et al., GCN Circ. 15505
Not found
 
 
).  The BAT ground-calculated position is
RA, Dec = 278.937, -12.026 deg which is 
  RA(J2000)  =  18h 35m 44.8s 
  Dec(J2000) = -12d 01' 34.8" 
with an uncertainty of 3.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 50%.

The mask-weighted light curve shows two weak peaks at T-20 sec and 
T+50 sec.  The emission starts at T-100 sec, and ends at T+80 sec.  
T90 (15-350 keV) is 131 +- 20 sec (estimated error including systematics).

The time-averaged spectrum from T-71.1 to T+64.3 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.94 +- 0.50.  The fluence in the 15-150 keV band is 5.7 +- 1.5 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+49.40 sec in the 15-150 keV band
is 0.5 +- 0.2 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/578227/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 1.92.  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 (e.g. Sakamoto et al. 2008, ApJ, 679, 570).