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

The upcoming Imaging X-ray Polarimetry Explorer (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 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.