Abstract
The effect of vacuum birefringence is one of the first predictions of quantum electrodynamics (QED): the presence of a charged Dirac field makes the vacuum birefringent when threaded by magnetic fields. This effect, extremely weak for terrestrial magnetic fields, becomes important for highly magnetized astrophysical objects, such as accreting black holes. In the X-ray regime, the polarization of photons traveling in the magnetosphere of a black hole is not frozen at emission but is changed by the local magnetic field. We show that, for photons traveling along the plane of the disk, where the field is expected to be partially organized, this results in a depolarization of the X-ray radiation. Because the amount of depolarization depends on the strength of the magnetic field, this effect can provide a way to probe the magnetic field in black-hole accretion disks and to study the role of magnetic fields in astrophysical accretion in general.
Highlights
In the theory of accretion disks around black holes and astrophysical accretion in general, magnetic fields play a crucial role
Before calculating the evolution of the photon polarization for a defined structure of the magnetic field, it is interesting to look at the quantity called the polarization-limiting radius (PLR)
To better understand how vacuum birefringence affects the polarization of photons traveling through the black hole magnetosphere, we assume a simple structure for the magnetic field threading the accretion disk, and we study how the polarization changes for photons traveling parallel to the disk plane
Summary
In the theory of accretion disks around black holes and astrophysical accretion in general, magnetic fields play a crucial role. From the analysis of the spectra of two Galactic stellar-mass black holes, Miller et al [5,6,7] showed that a wind is generated by magnetic processes as close as 850 GM/c2 to the hole. They obtained an estimate of the strength of the magnetic field when a certain magnetic process is assumed [7]. We describe how X-ray polarization measurements from black-hole accretion disks could provide a way to probe, for the first time, the strength and structure of the magnetic field close to the event horizon
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