T. Sakamoto, S. Barthelmy, N. Gehrels, J. Cummings, A. Parsons,
J. Tueller (GSFC), E. Fenimore, D. Palmer (LANL), D. Hullinger,
C. Markwardt (GSFC/UMD), H. Krimm (GSFC/USRA), G. Sato (ISAS),
M. Suzuki (Saitama)
With the further analysis of the BAT spectral data of GRB050215b,
we confirmed the nature of X-ray flash for GRB050215b.
If we used the best fit Epeak energy of 17.6 keV derived from the HETE
data (Nakagawa et al, Circ 3053), the transition energy between the
lower energy cutoff power-law component and the higher energy power-law
component of the Band function (Band et al. 1993) will be 26.4 keV if we
assume alpha=-1 and beta=-2.5. Based on this estimation, we tried to fit
the BAT spectrum only using above 30 keV to see the evidence of the
power-law slope of steeper than -2.
With this fit, we obtained the best fit power-law index of -2.99
(-1.02/+0.77; 90% confidence). The significance in the power-law
index of steeper than -2.0 is 97%. This is a strong indication
that BAT observed the higher portion of the Band function, and the
Epeak of GRB050215b is indeed below 30 keV.