  In earlier works, we had shown that the observed soft lags and r.m.s versus
energy of the lower kHz QPO of neutron star binaries can be explained in the
framework of a thermal Comptonization model. It was also shown that such an
interpretation can provide estimates of the size and geometry of the
Comptonizing medium. Here we study the dependence of these estimates on the
time-averaged spectral model assumed and on the frequency of the QPO. We use
the high quality time lag and r.m.s obtained during March 3rd 1996 observation
of 4U 1608-52 by RXTE as well as other observations of the source at different
QPO frequencies where a single time-lag between two broad energy bands have
been reported. We compare the results obtained when assuming that the
time-averaged spectra are represented by the spectrally degenerate "hot" and
"cold" seed photon spectral models. We find that for the "hot" seed photon
model the medium size is in the range of 0.3-2.0 kms and the size decreases
with increasing QPO frequency. On the other hand for the "cold" seed photon
model the range for the sizes are much larger 0.5-20 kms and hence perhaps show
no variation with QPO frequency. Our results emphasis the need for broad band
spectral information combined with high frequency timing to lift this
degeneracy. We further show that the r.m.s as a function of energy for the
upper kHz QPO is similar to the lower one and indeed we find that the driver
for this QPO should be temperature variations of the corona identical to the
lower kHz QPO. However, the time lag reported for the upper kHz QPO is hard,
which if confirmed, would challenge the simple Comptonization model presented
here. It would perhaps imply that reverberation lags are also important and/or
the dominate spectral component is not a single temperature medium but a
multi-temperature complex one.
