Compton scattering of polarized radiation in two-phase accretion disks in active galactic nuclei

We present results of radiative transfer calculations of Compton scattering of polarized radiation in a corona above an accretion disk. The corona has a plane-parallel geometry and is assumed to be homogeneous with a fixed temperature. We employ a Feautrier method to calculate the radiative transfer, incorporating the full relativistic, polarized Compton redistribution matrix. This allows us to explore a broad range of parameter space, in particular large optical depths and/or low corona temperatures, for which the iterative scattering method is very inefficient. Compared with Thomson scattering, Compton scattering generally produces smaller polarizations for featureless spectra. Distinct frequency-dependent patterns in polarization are produced near lines and edges, which can be used as diagnostics for the corona. We discuss the physics that gives rise to these results. Compton scattering can smear out Lyman edges in total flux and at the same time produce large increases in polarization. This might help explain the rises in polarization observed near the Lyman limit in some quasars, although in general Compton scattering does not appear able to produce polarization changes that are as steep as observed.