Observable gravitational effects on polarised radiation coming from near a black hole

THE effects of general relativity on the degree and plane of linear polarisation of X rays emitted by an accretion disk orbiting a black hole (such as Cyg X-1 might contain), were described in a previous letter¹ where we showed that the effects were large. Using a constant of motion along null geodesies in the Kerr metric, discovered by Penrose and Walker², we have since been able to calculate analytically polarisation rotations along any null ray. We present here more extensive results obtained by this method, for two different kinds of disk model and for different black hole angular momenta.