X-ray Polarization from Black Holes in the Thermal State

TL;DR

This paper introduces advanced calculations of X-ray polarization from black hole accretion disks, incorporating returning radiation effects to improve black hole spin measurements and disk flow analysis.

Contribution

It presents a novel Monte-Carlo ray-tracing method in general relativity that includes returning radiation scattering in polarization predictions.

Findings

Scattered radiation is highly polarized and perpendicular to direct radiation.

Returning radiation significantly influences polarization spectra.

Method enhances black hole spin and accretion disk flow diagnostics.

Abstract

We present new calculations of X-ray polarization from black hole accretion disks in the thermally-dominated state, using a Monte-Carlo ray-tracing code in full general relativity. In contrast to many previously published studies, our approach allows us to include returning radiation that is deflected by the strong-field gravity of the BH and scatters off of the disk before reaching a distant observer. Although carrying a relatively small fraction of the total observed flux, the scattered radiation tends to be highly polarized and in a direction perpendicular to the direct radiation. We show how these new features of the polarization spectra may be developed into a powerful tool for measuring black hole spin and probing the gas flow in the innermost disk.