Spectral and polarization properties of black hole accretion disc emission: including absorption effects

TL;DR

This paper investigates how absorption effects influence the polarization and spectral properties of radiation from black hole accretion discs, highlighting the importance of ionization and absorption in interpreting observational data.

Contribution

It extends previous models by incorporating absorption effects through ionization structure calculations, providing a more comprehensive understanding of polarization in black hole accretion discs.

Findings

Absorption can significantly increase the polarization degree.

Ionization state affects the polarization properties.

Absorption effects are more prominent at low accretion rates and spins.

Abstract

The study of radiation emitted from black hole accretion discs represents a crucial way to understand the main physical properties of these sources, and in particular the black hole spin. Beside spectral analysis, polarimetry is becoming more and more important, motivated by the development of new techniques which will soon allow to perform measurements also in the X- and {\gamma}-rays. Photons emitted from black hole accretion discs in the soft state are indeed expected to be polarized, with an energy dependence which can provide an estimate of the black hole spin. Calculations performed so far, however, considered scattering as the only process to determine the polarization state of the emitted radiation, implicitly assuming that the temperatures involved are such that material in the disc is entirely ionized. In this work we generalize the problem by calculating the ionization structure of a surface layer of the disc with the public code CLOUDY , and then by determining the polarization properties of the emerging radiation using the Monte Carlo code STOKES . This allows us to account for absorption effects alongside scattering ones. We show that including absorption can deeply modify the polarization properties of the emerging radiation with respect to what is obtained in the pure-scattering limit. As a general rule, we find that the polarization degree is larger when absorption is more important, which occurs e.g. for low accretion rates and/or spins when the ionization of the matter in the innermost accretion disc regions is far from complete.