Polarization properties of thermal accretion disk emission

I. Direct radiation

¹ INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
² Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Via Marzolo 8, 35131 Padova, Italy
³ Astronomical Institute of the Czech Academy of Sciences, Boční II 1401/1, 14100 Praha 4, Czech Republic
⁴ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
⁵ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 12 February 2025
Accepted: 27 October 2025

Abstract

The X-ray polarimetric observing window reopening is shedding new light on our current understanding of compact accreting sources. This is particularly true 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 toward 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 in either 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.