  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.
