The high-energy polarization-limiting radius of neutron star magnetospheres – I. Slowly rotating neutron stars

In the presence of strong magnetic fields, the vacuum becomes a birefringent medium. We show that this quantum electrodynamics effect decouples the polarization modes of photons leaving the neutron star (NS) surface. Both the total intensity and the intensity in each of the two modes are preserved along the path of a ray through the NS magnetosphere. We analyse the consequences that this effect has on aligning the observed polarization vectors across the image of the stellar surface to generate large net polarizations. In contrast to previous predictions, we show that the thermal radiation of NSs should be highly polarized even in the optical. When detected, this polarization will be the first demonstration of vacuum birefringence. It could be used as a tool to prove the high magnetic field nature of anomalous X-ray pulsars (AXPs) and it could also be used to constrain physical NS parameters, such as R/M, to which the net polarization is sensitive.