Polarization Structures in the Thomson-Scattered Emission Lines in Active Galactic Nuclei

Abstract

A line photon incident in an electron-scattering medium is transferred in a diffusive way both in real space and in frequency space, and the mean number of scatterings changes as the wavelength shifts from the line center. This leads to the profile broadening and polarization dependence on the wavelength shift as a function of the Thomson optical depth $τ_T$. We find that the polarization of the Thomson-scattered emission lines has a dip around the line center when $τ_T$ does not exceed a few. Various structures such as the polarization flip are also seen. An application to an ionized halo component surrounding the broad emission line region in active galactic nuclei is considered and it is found that the polarization structures may still persist. Brief discussions on observational implications are given.