  We develop a simple, time-dependent Comptonization model to probe the origins
of spectral variability in accreting neutron star systems. In the model, soft
``seed photons'' are injected into a corona of hot electrons, where they are
Compton upscattered before escaping as hard X-rays. The model describes how the
hard X-ray spectrum varies when the properties of either the soft photon source
or the Comptonizing medium undergo small oscillations. Observations of the
resulting spectral modulations can determine whether the variability is due to
(i) oscillations in the injection of seed photons, (ii) oscillations in the
coronal electron density, or (iii) oscillations in the coronal energy
dissipation rate. Identifying the origin of spectral variability should help
clarify how the corona operates and its relation to the accretion disk. It will
also help in finding the mechanisms underlying the various quasi-periodic
oscillations (QPO) observed in the X-ray outputs of many accreting neutron star
and black hole systems. As a sample application of our model, we analyze a
kilohertz QPO observed in the atoll source 4U~1608-52. We find that the QPO is
driven predominantly by an oscillation in the electron density of the
Comptonizing gas.

Astrophysics