Strongly Magnetized Sources: QED and X-ray Polarization

Abstract

Radiative corrections of quantum electrodynamics cause a vacuum threaded by a magnetic field to be birefringent. This means that radiation of different polarizations travels at different speeds. Even in the strong magnetic fields of astrophysical sources, the difference in speed is small. However, it has profound consequences for the extent of polarization expected from strongly magnetized sources. We demonstrate how the birefringence arises from first principles, show how birefringence affects the polarization state of radiation and present recent calculations for the expected polarization from magnetars and X-ray pulsars.

Axions: From Magnetars and Neutron Star Mergers to Beam Dumps and BECs

“…For photon energies lower than the electron cyclotron energy ∼ 11.6(B/10 12 G) keV, astrophysical models predict that the X-mode opacity is suppressed with respect to the O-mode. Thus, the thermal radiation is almost completely polarized in the X-mode [146,147]. Remarkably, resonant inverse Compton upscattering, which is the putative astrophysical model underlying the hard emission, also predicts a strongly polarized X-mode emission [148].…”

“…The adiabatic tracking of the polarization vector continues up to the polarization radius r P L . This leads to astrophysical predictions of polarization fractions as high as 40% to 80% at the detector [146]. Therefore, not only is the astrophysical emission X-mode, the birefringence effect in the magnetosphere near magnetars "locks in" this mode as far as detection is concerned.…”

“…This "locking in" is especially efficient in magnetars with their strong magnetic fields. The radius of polarization is given by [146]…”

X-ray polarization signals from magnetars with axion-like-particles

“…The adiabatic tracking of the polarization vector continues up to the polarization radius. For a bipolar magnetic field, the radius is given by [21] r P L = α 45 ν c The observed X-mode polarization will be perpendicular to the magnetic field direction at the polarization radius. Since the radius is large, the polarization will not track the magnetic field structure near the magnetar, but rather, will be oriented perpendicularly to the direction of the magnetic axis.…”

“…We thus note that the effect of QED is to enhance the observed degree of polarization up to 70% or even more in the X-mode. Thus, a large amount of polarization can be an indicator of non-linear QED effects [21].…”