X-Ray Polarization Properties in the Two-Phase Model for AGN

Summary

Abstract

The polarization properties of a two-phase model, recently proposed to explain the X-ray emission of Active Galactic Nuclei, have been calculated for different values of the model parameters. An important signature of the model is the orthogonality between the UV/soft X-ray and hard X-ray polarization.

Recently, a two-phase model in which hot, thermal electrons in an optically thin layer comptonize the soft photons coming from an underlying cold, optically thick accretion disc, has been proposed to explain the X-ray emission of Active Galactic Nuclei.

Assuming a plane-parallel geometry, and isotropic and unpolarized disc thermal radiation, we have calculated the polarization properties as a function of the energy and of the inclination angle, for different values of τ₀, the optical depth of the hot phase (which, in the adopted model, is related to the electron temperature). This was done by solving the well-known equation of radiative transfer by separating the different scattering orders. The polarization of the X-rays reflected from the disc has also been taken into account. In the figure we show the degree of polarization as a function of the energy for different values of the inclination angle (at the two extremes of the energy range ∣P∣ increases with it). The assumed energy shape of the thermal radiation is a black-body with T=50eV. Note that the hard X-rays have a negative polarization (i.e. the polarization vector lies in the meridian plane), while the polarization of the UV/soft X-rays is positive (i.e. the polarization vector is perpendicular to the meridian plane).