General Relativistic Magnetoionic Theory
  We have developed methods for tracing rays and performing radiative transfer through a magnetoactive plasma in a general relativistic environment. The two electromagnetic plasma modes propagate differently due to a combination of dispersive and gravitational effects. We have found that, when given an appropriate environment surrounding the central black hole, it is indeed possible to generate a significant degree of circular polarisation without an appreciable amount of linear polarisation due to these effects alone. 
Avery Broderick and Roger Blandford (Theoretical Astrophysics,   Caltech)