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

It is shown that the delay (of the order of ms) between the quasi-periodic oscillations (QPO), observed from Cyg X-2 and GX 5-1, 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 it is shown 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. (author)