Polarization Variability of Active Galactic Nuclei and X‐Ray Binaries

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Abstract
In the innermost part of an accretion disk around a black hole, electron scattering could provide the dominant opacity, and so the X-ray radiation originating from this hot region is expected to be partially linearly polarized. If strong magnetic fields are present on or above the disk, then synchrotron radiation from electrons may also contribute to the polarization, although different orientations of the magnetic field and Faraday rotation might reduce the effect. Both observational and theoretical studies suggest that the inner disk region is unstable and could appear "clumpy." In this paper we investigate polarization features due to polarized orbiting clumps around a black hole. It is found that, in contrast to the Newtonian case, rapid polarization variability can be produced by those regions emitting extra radiation, and that the variability amplitudes of both the degree of polarization and the angle of the polarization plane are energy dependent, i.e., the polarization variability amplitudes are larger at higher energy. This feature will not appear if the central object is not gravitationally strong, even when polarized clumps rotate around the object, and this trend depends only weakly on the local physics, such as the specific polarization mechanism or optical depth of the sources. Energy-dependent polarization variability is a direct result of near-field bending of light rays by the central black hole, and it is unique to black hole systems involving accretion disks. Since accretion disks around black holes are expected in both active galactic nuclei and X-ray binaries, we look for future X-ray polarimetry missions to confirm our prediction of this phenomenon.