Fourier analysis of X-ray pulsar data reveals the presence of time
lags between hard and soft channels in millisecond pulsars. There is
currently no consistent theoretical explanation for this effect based
on a fundamental physical model for pulsar sources. In the proposed
research, a new theoretical model is developed from first principles
based on the bulk and thermal Comptonization occurring in the gas
inside the accretion column above one (or both) of the magnetic poles
on a rotating neutron star. The model utilizes a combination of
Fourier and Laplace transformation in order to obtain quantitative
predictions for the time lags. This approach will be used to make
predictions about the possible presence of time lags in the spectra of
bright pulsars such as Her X-1. Theoretical interpretation of the time
lags can provide detailed information about the size and properties of
the scattering plasma and also the spatial density profile of the
scattering electrons.