Clumpy accretion flows in active galactic nuclei: X-ray variability and spectral properties

We discuss the fueling of the central black hole in active galactic nuclei (AGN) in the framework of a clumpy accretion flow. Shocks between elements (clumps) forming the accretion flow are at the origin of the observed radiation. We expect a cascade of shocks, with optically thick shocks giving rise to optical/UV emission and optically thin shocks accounting for the X-ray emission. We associate the characteristic X-ray variability time scale derived within our model with the break time scale observed in the X-ray power spectral density (PSD). The predicted dependence of the variability time scale on black hole mass and accretion rate is in agreement with the observational relation obtained by McHardy et al. (2006). We further discuss X-ray spectral properties, in particular we compare the predicted hard X-ray power law slope with observations. Model results are able to reproduce the range of photon index typically measured in Seyfert galaxies and may account for the observed correlation between photon index and normalized accretion rate.