Covariant magnetoionic theory - II. Radiative transfer

Abstract

Accretion on to compact objects plays a central role in high-energy astrophysics. In these environments, both general relativistic and plasma effects may have significant impacts upon the spectral and polarimetric properties of the accretion flow. In Paper I we presented a fully general relativistic magnetoionic theory, capable of tracing rays in the geometric optics approximation through a magnetized plasma in the vicinity of a compact object. In this paper we discuss how to perform polarized radiative transfer along these rays. In addition we apply the formalism to a barotropic thick-disc model, appropriate for low-luminosity active galactic nuclei. We find that, when the observation frequency is near the plasma and cyclotron frequencies in the innermost portions of the accretion flow, it is possible to generate large fractional polarizations, even when the emission mechanism is unpolarized. This has implications for accreting systems ranging from pulsars and X-ray binaries to active galactic nuclei.