TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4403
SUBJECT: GRB 051227: Joint Swift-HETE Spectral Analysis 
DATE:    05/12/28 03:50:13 GMT
FROM:    Don Lamb at U.Chicago  <lamb@oddjob.uchicago.edu>

GRB 051227: Joint Swift-HETE Spectral Analysis

T. Sakamoto, J-L. Atteia, G. Ricker, 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, 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, J. Doty, G. Prigozhin, R. Vanderspek,
J. Villasenor, J. G. Jernigan, A. Levine, G. Azzibrouck, J. Braga,
R. Manchanda, G. Pizzichini, and S. Gunasekera, 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;

L. Barbier, S. Barthelmy, J. Cummings, N. Gehrels, and J. Norris, on
behalf of the Swift BAT team;

report:

The HETE FREGATE instrument detected the initial peak of GRB 051227
(Swift Trigger 174738; Barbier, et al., GCN 4397) as an untriggered
event.

Carrying out a fit to the FREGATE spectral data alone, the spectrum 
of the initial peak is adequately fit by a power-law (PL) model, with
photon index alpha = 1.3 +0.4/-0.6 (90% confidence level).  This
confirms the result reported by Hullinger et. al (GCN 4400).

Carrying out a joint fit to the BAT plus FREGATE spectral data, the
best-fit cutoff PL model yields an improvement in ch^i2 over the
best-fit PL model of 5.2 for one additional parameter, corresponding 
to a significance of 2.3E-02.  Thus the joint spectral fit provides
evidence of curvature in the spectrum.

Adopting the best-fit cutoff PL model yields a best-fit value of 
Epk = 100 +219/-41.3 keV (90% confidence level) and a fluence ratio 
S(2 keV-30 keV)/S(30 keV-400 keV) = 0.19.  The event is therefore a 
hard GRB.