QED and X-ray polarization from neutron stars and black holes

The emission from accreting black holes and neutron stars, as well as from the highly magnetized neutron stars called magnetars, is dominated by X-rays. For this reason, spectral and timing studies in the X-rays have been extremely successful in broadening our understanding of compact objects in the past few decades. Soon, a new observational window will open on compact objects: X-ray polarimetry. In this work, I explore how polarized light is generated in black-hole accretion disks, magnetar atmospheres and magnetospheres and in the accretion region of X-ray pulsars. In the different chapters, I show how the polarization signal is sensitive to several unknowns in our theoretical models: the geometry of accretion in X-ray pulsars, the strength and structure of the magnetic field threading accretion disks around black holes, the process of the non-thermal emission in magnetars. For this reason, the future X-ray polarimetry missions will be extremely helpful in constraining our theoretical models. Furthermore, the polarization emission will provide, for the first time, a test of one of the first theoretical predictions of quantum electrodynamics: vacuum birefringence. In this work, I show how this effect, previously considered only for neutron stars, plays a crucial role for black holes as well.Science, Faculty ofPhysics and Astronomy, Department ofGraduat