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We study the energy dependent photon variability from a thermal Comptonizingplasma that is oscillating at kHz frequencies. In particular, we solve thelinearised time dependent Kompaneets equation and consider the oscillatoryperturbation to be either in the soft photon source or in the heating rate ofthe plasma. For each case, we self consistently consider the energy balance ofthe plasma and the soft photon source. The model incorporates the possibilityof a fraction of the Comptonized photons impinging back into the soft photonsource. We find that when the oscillation is due to the soft photon source, thevariation of the fractional root mean sqaure (r.m.s) is nearly constant withenergy and the time-lags are hard. However, for the case when the oscillationis due to variation in the heating rate of the corona, and when a significantfraction of the photons impinge back into the soft photon source, the r.m.sincreases with energy and the time lags are soft. As an example, we compare theresults with the $\sim 850$ Hz oscillation observed on March 3, 1996 for 4U1608-52 and show that both the observed soft time-lags as well as the r.m.sversus energy can be well described by such a model where the size of theComptonizing plasma is $\sim 1$ km. Thus, modelling of the time lags as due toComptonization delays, can provide tight constraints on the size and geometryof the system. Detailed analysis would require well constrained spectralparameters.

Energy Dependent Time Delays of kHz Oscillations due to Thermal Comptonization