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In earlier works, it was shown that the energy dependent soft time lagsobserved in kHz QPOs of neutron star low mass X-ray binaries (LMXBs) can beexplained as being due to Comptonization lags provided a significant fraction($\eta \sim 0.2$ - $0.8$) of the Comptonized photons impinge back into the softphoton source. Here we use a Monte Carlo scheme to verify if such a fraction isviable or not. In particular we consider three different Comptonizing mediumgeometries: (i) a spherical shell, (ii) a boundary layer like torus and (iii) acorona on top of an accretion disk. Two set of spectral parameterscorresponding to the 'hot' and 'cold' seed photon models were explored. Thegeneral result of the study is that for a wide range of sizes, the fractionlies within $\eta \sim 0.3$ - $0.7$, and hence compatible with the rangerequired to explain the soft time lags. Since there is a large uncertainty inthe range, we cannot concretely rule out any of the geometries or spectralmodels, but the analysis suggests that a boundary layer type geometry with a'cold' seed spectral model is favoured over an accretion corona model. Betterquality data will allow one to constrain the geometry more rigorously. Ourresults emphasise that there is significant heating of the soft photon sourceby the Comptonized photons and hence this effect needs to be taken into accountfor any detailed study of these sources.

A Monte Carlo estimate for the fraction of thermal Comptonized photons that impinge back on the soft source in neutron star LMXBs