X-ray Polarization from Accreting Black Holes: II. The Thermal State

TL;DR

This study models X-ray polarization from accreting black holes in the thermal state, highlighting the impact of returning radiation on polarization signals and its potential for measuring black hole properties.

Contribution

It introduces a full general relativistic Monte Carlo method to include returning radiation effects in polarization calculations, enhancing black hole spin and accretion disk diagnostics.

Findings

Returning radiation significantly influences polarization at high energies.

High spin black holes show a 90-degree rotation in polarization angle.

Polarization can help measure black hole spin, mass, inclination, and distance.

Abstract

We present new calculations of X-ray polarization from black hole (BH) 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. For moderately large spin parameters (a/M >~ 0.9), this scattered returning radiation dominates the polarization signal at energies above the thermal peak, giving a net rotation in the polarization angle of 90 deg. We show how these new features of the polarization spectra from BHs in the thermal state may be developed into a powerful tool for measuring BH spin and probing the gas flow in the innermost disk. In addition to determining the emission profile, polarization observations can be used to constrain other properties of the system such as BH mass, inclination, and distance. New instruments currently under development should be able to exploit this tool in the near future.