Phase Difference and Coherence as Diagnostics of Accreting Compact Sources

We present calculations of the time lags and the coherence function of X-ray photons for a novel model of radiation emission from accretion-powered high-energy sources. Our model involves only Comptonization of soft photons injected near the compact object in an extended but nonuniform atmosphere around the compact object. Our results show that this model produces time lags between the hard and soft bands of the X-ray spectrum that increase with Fourier period, in agreement with recent observations; it also produces a coherence function equal to one over a wide range of frequencies if the system parameters do not change significantly during the observation, also in agreement with the limited existing observations. We explore various conditions that could affect coherence functions. We indicate that measurements of these statistical quantities could provide diagnostics of the radial variation of the density of this class of sources.