Previous analytical results on the temporal behavior of emissions from Comptonization plasma are obtained for nonrelativistic diffusion approximation and are therefore applicable only to plasma clouds, with low temperatures and large optical radii. By introducing the concept of varied mean free path and corresponding parameter, we are able to extend the analytical results to Comptonization clouds with small optical radii. We also compare the results of Monte Carlo calculations with the analytical formulae for light curves in given energy ranges and show that these formulae cannot be used to analyze the temporal behavior of the emission observed from black hole candidates or other sources if the energy spectra of these sources indicate the existence of clouds with temperature approx> 25 keV and radius ～1 Thomson mean free path. We performed Monte Carlo calculations based on a more realistic model and obtain the light curves in various energy ranges used in previous analyses. We performed Fourier analysis on these light curves and obtained the phase lags of harder X-rays with respect to softer ones as well as power spectrum density. We investigate the effects of various conditions of plasma clouds and source photons upon the phase lag and power spectrum density as functions of Fourier frequency. We discuss the implication of these results when compared to observations by instruments with high time resolution.
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