Journal Article

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

Eric Agol, Omer Blaes and Cristian Ionescu-Zanetti

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 293, issue 1, pages 1-17
Published in print January 1998 | ISSN: 0035-8711
Published online January 1998 | e-ISSN: 1365-2966 | DOI:
Polarization from magnetized accretion discs - II. The effects of absorption opacity on Faraday rotation

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Equipartition magnetic fields can dramatically affect the polarization of radiation emerging from accretion disc atmospheres in active galactic nuclei. We extend our previous work on this subject by exploring the interaction between Faraday rotation and absorption opacity in local, plane-parallel atmospheres with parameters appropriate for accretion discs. Faraday rotation in pure scattering atmospheres acts to depolarize the radiation field by rotating the polarization planes of photons after last scattering. Absorption opacity in an unmagnetized atmosphere can increase or decrease the polarization compared to the pure scattering case, depending on the thermal source function gradient. Combining both Faraday rotation and absorption opacity, we find the following results. If absorption opacity is much larger than scattering opacity throughout the atmosphere, then Faraday rotation generally has only a small effect on the emerging polarization because of the small electron column density along a photon mean free path. However, if the absorption opacity is not too large and it acts alone to increase the polarization, then the effects of Faraday rotation can be enhanced over those in a pure scattering atmosphere. Finally, while Faraday rotation often depolarizes the radiation field, it can in some cases increase the polarization when the thermal source function does not rise too steeply with optical depth. We confirm the correctness of the analytic calculation by Silant'ev of the high magnetic field limit of the pure scattering atmosphere, which we incorrectly disputed in our previous paper.

Keywords: accretion, accretion discs; magnetic fields; polarization; galaxies: active

Journal Article.  0 words. 

Subjects: Astronomy and Astrophysics

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