Systematic analysis of polarization for thermal radiation of neutron star focusing on mode conversion

Akihiro Yatabe, Shoichi Yamada

Polarization in soft x-ray will be observed in the near future. It is a new observation quantity in this energy range and it is thought that observations of x-ray polarization will develop x-ray astronomy drastically. One of the targets is a magnetar, which is a neutron star with a notably strong magnetic field. Thermal radiation of a magnetar usually dominates over non-thermal radiation for soft x-ray and is thought to be linearly polarized because of different opacities for two polarization modes of photons in the magnetized atmosphere and dielectric properties of a vacuum in the magnetosphere affected by vacuum polarization. To analyze observational results efficiently in the future, it is important to be able to compute polarization observable quantities systematically in preparation to future observations. Although a previous study Taverna et al. (2015) considered this problem and developed a method to estimate polarization properties systematically, it ignored conversion of photon polarization mode in the atmosphere, which may cause discontinuous changes in polarization angles, one of the polarization observables. So, we revisit this problem especially focusing mode conversion. We compute polarization angles and polarization fractions, then consider polarization properties for different magnetic fields, radii of the emission region and so on. It is confirmed in our calculation that mode conversion is important when the magnetic field is relatively weak and that the radius affects to polarization fractions averaged over a spin period.