GCN Circular 4145

GRB 051022:  Spectral Analysis Indicates That The Spectrum Is Absorbed

J. Doty, G. Ricker, J-L. Atteia, N. Kawai, D. Lamb, and S. Woosley, on 
behalf of the HETE Science Team; 

M. Arimoto, T. Donaghy, E. Fenimore, M. Galassi, C. Graziani, 
N. Ishikawa, A. Kobayashi, J. Kotoku, M. Maetou, M. Matsuoka, 
Y. Nakagawa, T. Sakamoto, R. Sato, T. Shimokawabe, Y. Shirasaki, 
S. Sugita, M. Suzuki, T. Tamagawa, K. Tanaka, and A. Yoshida, on behalf 
of the HETE WXM Team; 

N. Butler, G. Crew, G. Prigozhin, R. Vanderspek, J. Villasenor, 
J. G. Jernigan, A. Levine, G. Azzibrouck, J. Braga, R. Manchanda, 
and G. Pizzichini, on behalf of the HETE Operations and HETE 
Optical-SXC Teams; 

M. Boer, J-F Olive, J-P Dezalay, and K. Hurley, on behalf of the HETE 
FREGATE Team; 

report:

We have performed a spectral analysis of the WXM and FREGATE spectral
data for GRB 051022.  Fitting a power-law times exponential (PLE) model
jointly to the WXM and FREGATE spectral data yields a chi^2 of 185 for
139 DOF, while fitting a PLE model with absorption yields a chi^2 of
131 for 138 DOF.  Fitting a Band model jointly to the WXM and FREGATE
spectral data also yields a chi^2 of 131 for 138 DOF.  These results
show that the data strongly favor a PLE model with absorption over a
PLE model alone, and that a PLE model with absorption gives a 
satisfactory fit to the data.  They also show that the data do not 
request a Band model with absorption.

The best-fit spectral parameters for the PLE model with absorption are
alpha = -1.22+/-0.02, E_peak = 306 + 31/-26 keV, and n_H = 1.7 � 0.4
x 10^22 atoms cm^-2.  The fluence is (2.2 � 0.02) x 10^-5 erg cm^-2 
in the 2-30 keV energy band and (1.4 � 0.02) x 10^-4 erg cm^-2 in the
30-400 keV energy band.  The ratio log(S_x/S_g) = -1.8, so GRB 051022 
is a "classical" hard GRB.