  We present the results of our study of the variability time scales of a
sample of 27 long Swift Gamma-Ray Bursts (GRBs) with known redshifts. The
variability time scale can help our understanding of fundamental GRB parameters
such as the initial bulk Lorentz factor and the characteristic size associated
with the emission region. Fast Fourier Transform (FFT) techniques were used to
extract a noise threshold crossing frequency, which we associate with a
variability time scale. The threshold frequency appears to show a correlation
with the peak isotropic luminosity of GRBs.
