Systematic Analysis of the Effects of Mode Conversion on Thermal Radiation from Neutron Stars

TL;DR

This study systematically analyzes how mode conversion affects the polarization of soft X-ray radiation from magnetized neutron stars, highlighting its significance at different magnetic field strengths and implications for upcoming observations.

Contribution

It provides a detailed calculation of polarization changes due to mode conversion in neutron star atmospheres, considering various physical parameters and magnetic field strengths.

Findings

Mode conversion effects are significant at B ~ 10^{13} G.

Strongly magnetized neutron stars (B > 10^{14} G) are ideal for polarization detection.

Neutron stars with B ~ 10^{13} G can confirm mode conversion phenomena.

Abstract

In this paper, we systematically calculate the polarization in soft X-rays emitted from magnetized neutron stars, which are expected to be observed by the next-generation X-ray satellites. Magnetars are one of the targets for these observations. This is because thermal radiation is normally observed in the soft X-ray band, and it is thought to be linearly polarized because of different opacities for two polarization modes of photons in the magnetized atmosphere of neutron stars and the dielectric properties of the vacuum in strong magnetic fields. In their previous study, Taverna et al. illustrated how strong magnetic fields influence the behavior of the polarization observables for radiation propagating in vacuo without addressing a precise, physical emission model. In this paper, we pay attention to the conversion of photon polarization modes that can occur in the presence of an atmospheric layer above the neutron star surface, computing the polarization angle and fraction and systematically changing the magnetic field strength, radii of the emission region, temperature, mass, and radii of the neutron stars. We confirmed that if plasma is present, the effects of mode conversion cannot be neglected when the magnetic field is relatively weak, $B \sim 10^{13} \mathrm{G}$. Our results indicate that strongly magnetized ($B \gtrsim 10^{14} \mathrm{G}$) neutron stars are suitable to detect polarizations, but not-so-strongly magnetized ($B \sim 10^{13} \mathrm{G}$) neutron stars will be the ones to confirm the mode conversion.