On the role of strong gravity in polarization from scattering of light in relativistic flows

Abstract

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. Effects of special and general relativity are included by connecting a model of Beloborodov (1998) for the local polarization of scattered light with Abramowicz, Ellis & Lanza (1990) formalism for the particle motion near an ultra-compact star. 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. 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 travel time.