Polarized line emission from magnetized accretion flows

Abstract

We construct models of polarized line emission from magnetized accretion flows and investigate how line circular polarization can be used to infer the dynamics and the physics of the field-channelled flow in AM Herculis systems. We show that the line-polarization characteristics depend strongly on field and velocity gradients in the gas. We identify two regions of the flow which can contribute to the polarized line emission. The first is the funnelled flow out of the orbital plane along undistorted field lines. The second is the flow in the transition region closer to the orbital plane where the field lines are distorted, and the fluid motion changes from ballistic to corotation with the white dwarf. We argue that depending on field orientation and viewing aspect one or the other could dominate. In the high-inclination system AR UMa, the polarized line emission is dominated by field-channelled flow in the transition region. In contrast, in the low-inclination system V884 Her, the polarized line emission appears to be dominated by funnel flow along undistorted field lines.