GRB 131120A

A. Maselli (INAF-IASFPA), S. T. Holland (STScI), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL), T. Sakamoto (AGU),
B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf
of the Swift Team:

At 14:37:56 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 131120A (trigger=578227).  Swift could not slew to the burst
due to a Sun constraint. 
The BAT on-board calculated location is 
RA, Dec 278.921, -11.997 which is 
   RA(J2000) = 18h 35m 41s
   Dec(J2000) = -11d 59' 49"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual for image triggers, the
immediately-available BAT lightcurve does not show strong
variation. 

Due to a Sun observing constraint, Swift cannot slew to the BAT
position until 15:39 UT on 2014 February 13. There will thus be no XRT
or UVOT data for this trigger before this time. 

Burst Advocate for this burst is A. Maselli (maselli AT ifc.inaf.it). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)

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
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 ).  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).