Time lag between hard and soft X-ray photons in QPO sources

We show that the delay (of the order of ms) between the quasi-periodic oscillations (QPO), observed from CygX-2 and GXS-1, in a high-energy and a low-energy photon pass-band, can be the result of Compton scattering of photons in a cloud of hot electron gas. This delay depends on the optical depth, electron temperature, and radius of the cloud, and on the spectrum of the input photons. Using Monte Carlo simulations we show how, for an assumed input spectrum of photons, the observed delay and X-ray spectrum can be used to constrain the size of the scattering cloud. We find that, in addition to scattering of an oscillating signal in a constant cloud, scattering of a constant source of input photons through a cloud with oscillating optical depth can also give rise to both QPO in the emergent photons, and a time lag between high-energy and low-energy photons.