Clumpy Accretion Flows in Active Galactic Nuclei

摘要

We discuss the fueling of the central black hole in active galactic nuclei 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 obtain X-ray to UV luminosity ratios (Lx/Luv) always smaller than unity and lower values in massive objects compared to less massive sources, consistent with the observed αox — luv anticorrelation. We derive a characteristic X-ray variability time scale within our model and compare it with the observed X-ray power spectral density break time scale. The predicted dependence of the vari-ability time scale on black hole mass and accretion rate is in agreement with the observational relation obtained by McHardy et al. (2006).