Polarization properties of thermal accretion disk emission. I. Direct radiation

TL;DR

This paper investigates how ionization states and relativistic effects influence the polarization of thermal emission from accretion disks around black holes, enhancing understanding of X-ray polarimetry in these systems.

Contribution

It combines ionization state modeling with relativistic propagation effects to analyze polarization properties of accretion disk emission, focusing on direct radiation.

Findings

Ionization state affects polarization degree and angle.

Relativistic effects modify polarization signals near black holes.

Analysis provides insights for interpreting X-ray polarimetric observations.

Abstract

The X-ray polarimetric observing window re-opening is shedding new light on our current understanding of compact accreting sources. This is true, in particular, for stellar-mass black hole sources observed in the thermally-dominated state, for which the polarization signal is expected to depend on the accretion disk inclination and the black hole spin. Two main effects determine the polarization properties of the accretion disk emission: the absorption and scattering processes occurring before the radiation leaves the disk atmosphere, and the relativistic effects influencing its propagation towards the observer at infinity. In this work, we investigate these effects together considering only the contribution of direct radiation. We analyze how the ionization state of the disk atmosphere, approximated with a constant-density surface layer assumed to be either in collisional ionization equilibrium or photoionization equilibrium, can influence the spectro-polarimetric properties of the radiation at the emitting disk surface. Subsequently we study how these are modified by the propagation in a strong gravitational field.