GCN Circular 7761

T. Sakamoto (GSFC/UMBC), S. D. Barthelmy (GSFC), W. Baumgartner (GSFC/UMBC),
J. Cummings (GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC),
D. Grupe (PSU), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD),
K. McLean (GSFC/UMD), D. Palmer (LANL), G. Sato (GSFC/ISAS),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-240 to T+962 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 080520 (trigger #312047)
(Grupe, et al., GCN Circ. 7753
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 ).  The BAT ground-calculated position is
RA, Dec = 280.179, -54.964 deg which is 
   RA(J2000)  =  18h 40m 42.9s 
   Dec(J2000) = -54d 57' 51.2" 
with an uncertainty of 2.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.
 
The mask-weighted light curve shows a single peak starting at T_zero and
ending at ~T+3 sec.  T90 (15-350 keV) is 2.8 +- 0.7 sec (estimated error
including systematics).
 
The time-averaged spectrum from T-0.3 to T+2.8 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.90 +- 0.51.  The fluence in the 15-150 keV band is 5.5 +- 1.4 x 10^-8 erg/cm2.
The 1-sec peak photon flux measured from T-0.31 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. 
 
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/312047/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 1.88.  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. ApJ, 679, 570).