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In earlier works, we had shown that the observed soft lags and r.m.s versusenergy of the lower kHz QPO of neutron star binaries can be explained in theframework of a thermal Comptonization model. It was also shown that such aninterpretation can provide estimates of the size and geometry of theComptonizing medium. Here we study the dependence of these estimates on thetime-averaged spectral model assumed and on the frequency of the QPO. We usethe high quality time lag and r.m.s obtained during March 3rd 1996 observationof 4U 1608-52 by RXTE as well as other observations of the source at differentQPO frequencies where a single time-lag between two broad energy bands havebeen reported. We compare the results obtained when assuming that thetime-averaged spectra are represented by the spectrally degenerate "hot" and"cold" seed photon spectral models. We find that for the "hot" seed photonmodel the medium size is in the range of 0.3-2.0 kms and the size decreaseswith increasing QPO frequency. On the other hand for the "cold" seed photonmodel the range for the sizes are much larger 0.5-20 kms and hence perhaps showno variation with QPO frequency. Our results emphasis the need for broad bandspectral information combined with high frequency timing to lift thisdegeneracy. We further show that the r.m.s as a function of energy for theupper kHz QPO is similar to the lower one and indeed we find that the driverfor this QPO should be temperature variations of the corona identical to thelower kHz QPO. However, the time lag reported for the upper kHz QPO is hard,which if confirmed, would challenge the simple Comptonization model presentedhere. It would perhaps imply that reverberation lags are also important and/orthe dominate spectral component is not a single temperature medium but amulti-temperature complex one.

Constraining the size of the Comptonizing medium by modeling the energy dependent time-lags of kHz QPOs of Neutron star system