The effects of magnetic fields, absorption, and relativity on the polarization of accretion disks around supermassive black holes

Polarization of radiation emerging from accretion disks in active galactic nuclei (AGN) may be strongly affected by Faraday rotation, absorption opacity, and relativistic effects. We calculate the polarization of disk atmospheres including successively: (1) electron scattering with weak magnetic fields; (2) absorption opacity; (3) both magnetic fields and absorption opacity. We then compute global disk models including relativistic effects.; We find the following results: (1) Faraday rotation by an equipartition magnetic field does not affect the polarization in the extreme ultraviolet, but depolarizes in the optical, which may help explain the low optical polarizations observed in QSO's. (2) Blueward of the Lyman edge, the polarization rises above the value expected from an electron scattering atmosphere. This arises because of the wavelength dependence of the absorption opacity combined with a source function in this region of the spectrum that has a steep variation with optical depth. We predict that the polarization should continue to rise blueward of the wavelength where the current observations end, that cooler spectra should be polarized more, and that the polarization angle should be parallel to the disk plane. (3) If absorption opacity dominates, 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. (4) Due to relativistic effects, the polarization rise blueward of the Lyman edge is reduced in magnitude and blueshifted for edge-on disks, while the polarization is small for face-on disks. This probably means that standard thin accretion disks cannot explain the rise in polarization blueward of the Lyman edge seen in several quasars.; We also present spectropolarimetry of the broad H-alpha line in galaxy Arp 102B, and calculations of the polarization of stars during microlensing.