The Relative Importance of Faraday Rotation and QED Birefringence for the Linear Polarization of X-rays from Mass Accreting Black Holes

TL;DR

This paper assesses how Faraday rotation and QED birefringence influence the polarization measurements of X-rays from black holes, which is crucial for interpreting upcoming observational data from IXPE and XL-Calibur.

Contribution

It provides an estimation of the significance of Faraday rotation and QED birefringence effects on X-ray polarization from black holes, aiding in the interpretation of future measurements.

Findings

Faraday rotation can significantly alter polarization signals at certain energies.

QED birefringence effects are relevant for high-energy X-ray observations.

Implications for constraining black hole properties with upcoming missions.

Abstract

The upcoming IXPE (2-8 keV) and XL-Calibur (15-75 keV) missions will make it possible to measure the linear polarization of X-rays from mass accreting stellar mass black holes with unprecedented sensitivity, enabling the accurate measurement of percent-level and in some cases even sub-percent level polarization fractions. The measurements are expected to constrain the spins, inclinations, and the structure of the accretion flows of the observed black holes. The effects of Faraday rotation and birefringence of the Quantum Electrodynamics (QED) vacuum may impact the observable polarization fractions and angles, complicating the interpretation of the results. We estimate the importance of both effects for X-rays from stellar mass and supermassive black holes and discuss the implications of the results for the upcoming IXPE and XL-Calibur observations.