  This paper presents a study on the spectral evolution of gamma-ray burst
(GRB) prompt emissions observed with the Suzaku Wide-band All-sky Monitor
(WAM). By making use of the WAM data archive, 6 bright GRBs exhibiting 7
well-separated fast-rise-exponential-decay (FRED) shaped light curves are
presented and the evaluated exponential decay time constants of the
energy-resolved light curves from these FRED peak light curves are shown to
indicate significant spectral evolution. The energy dependence of the time
constants is well described with a power-law function tau(E) ~ E^gamma, where
gamma ~ -(0.34 +/- 0.12) in average, although 5 FRED peaks show consistent
value of gamma = -1/2 which is expected in synchrotron or inverse-Compton
cooling models. In particular, 2 of the GRBs were located with accuracy
sufficient to evaluate the time-resolved spectra with precise energy response
matrices. Their behavior in spectral evolution suggests two different origins
of emissions. In the case of GRB081224, the derived 1-s time-resolved spectra
are well described by a blackbody radiation model with a power-law component.
The derived behavior of cooling is consistent with that expected from radiative
cooling or expansion of the emission region. On the other hand, the other 1-s
time-resolved spectra from GRB100707A is well described by a Band GRB model as
well as with the thermal model. Although relative poor statistics prevent us to
conclude, the energy dependence in decaying light curve is consistent with that
expected in the former emission mechanism model.
