Polarization from magnetized accretion discs: II. The effects of absorption opacity on Faraday rotation

摘要

Equipartition magnetic fields can dramatically affect the polarization ofradiation emerging from accretion disk atmospheres in active galactic nuclei.We extend our previous work on this subject by exploring the interactionbetween Faraday rotation and absorption opacity in local, plane-parallelatmospheres with parameters appropriate for accretion discs. Faraday rotationin pure scattering atmospheres acts to depolarize the radiation field byrotating the polarization planes of photons after last scattering. Absorptionopacity in an unmagnetized atmosphere can increase or decrease the polarizationcompared to the pure scattering case, depending on the thermal source functiongradient. Combining both Faraday rotation and absorption opacity, we find thefollowing results. If absorption opacity is much larger than scattering opacitythroughout the atmosphere, then Faraday rotation generally has only a smalleffect on the emerging polarization because of the small electron columndensity along a photon mean free path. However, if the absorption opacity isnot too large and it acts alone to increase the polarization, then the effectsof Faraday rotation can be enhanced over those in a pure scattering atmosphere.Finally, while Faraday rotation often depolarizes the radiation field, it canin some cases increase the polarization when the thermal source function doesnot rise too steeply with optical depth. We confirm the correctness of theSilant'ev (1979) analytic calculation of the high magnetic field limit of thepure scattering atmosphere, which we incorrectly disputed in our previouspaper.