2006MNRAS.365..813H

We study linear polarization due to scattering of light on a cloudlet of particles, taking into account the radiation drag and the gravitational pull exerted on them by a central body. The effects of special and general relativity are included by connecting a model of Beloborodov for the local polarization of scattered light with Abramowicz et al. formalism for the particle motion near an ultracompact star. The compactness of the central body and its luminosity are two critical parameters of the model. We discuss the polarization magnitude of photons, which are Thomson-scattered into direct and higher-order images. The importance of the latter is only moderate under typical conditions, but they may give rise to distinct features, which we explore in terms of a toy model. The scattered signal exhibits variations of intensity and of polarization with mutual time-lags depending on the beaming/focusing effects and the light traveltime.