  We determined the characteristic variability time scales (\Delta t_p) of 410
bright and long GRBs, by locating the peaks of their Power Density Spectra,
defined and calculated in the time domain. We found that the averaged
variability time scale decreases with the peak flux. This is consistent with
the time-dilation effect expected for the cosmological origin of GRBs. We also
found that the occurrence distribution of the characteristic variability time
scale shows bimodality, which might be interpreted as that the long GRB sample
is composed of two sub-classes with different variability time scales. However,
we found no difference for some other characteristics of these two sub-classes.
