Polarization signatures of strong gravity in active galactic nuclei accretion discs

The effects of strong gravity on the polarization of the Compton reflection from an X-ray-illuminated accretion disc are studied. The gravitational field of a rotating black hole influences Stokes parameters of the radiation along the propagation to a distant observer. Assuming the lamp-post model, the degree and the angle of polarization are examined as functions of the observer's inclination angle, of the height of the primary source and of the inner radius of the disc emitting region. It is shown that polarimetry can provide essential information on the properties of black holes sources, and it is argued that time variation of the polarization angle is a strong signature of general-relativity effects. The expected polarization degree and angle should be detectable by new-generation polarimeters, such as that planned for the Xeus mission.