Polarization of light from warm clouds above an accretion disk: effects of strong-gravity near a black hole

Abstract

We study polarization from scattering of light on a cloud in radial motion along the symmetry axis of an accretion disk. Radiation drag from the disk and gravitational attraction of the central black hole are taken into account, as well as the effect of the cloud cooling in the radiation field. This provides us with a self-consistent toy-model for predicted lightcurves, including the linear polarization that arises from the scattering. Strong gravitational lensing creates indirect images; these are formed by photons that originate from the disk, get backscattered onto the photon circular orbit and eventually redirected towards an observer. Under suitable geometrical conditions the indirect photons may visibly influence the resulting magnitude of polarization and light-curve profiles. Relevant targets are black holes in active galactic nuclei and stellar-mass Galactic black-holes exhibiting episodic accretion/ejection events.