Polarization of neutron star surface emission: a systematic analysis

TL;DR

This paper develops an efficient method to compute the observed polarization of neutron star surface emission, accounting for quantum electrodynamics effects and magnetic field geometry, aiding interpretation of upcoming X-ray polarization observations.

Contribution

It introduces a general computational approach for polarization signals from neutron stars, incorporating vacuum polarization and magnetic field geometry effects.

Findings

Method effectively models polarization from entire neutron star surfaces.

Application to dipolar and twisted magnetic fields demonstrates versatility.

Results assist in interpreting future X-ray polarization measurements.

Abstract

New-generation X-ray polarimeters currently under development promise to open a new window in the study of high-energy astrophysical sources. Among them, neutron stars appear particularly suited for polarization measurements. Radiation from the (cooling) surface of a neutron star is expected to exhibit a large intrinsic polarization degree due to the star strong magnetic field ($\approx 10^{12}-10^{15}$ G), which influences the plasma opacity in the outermost stellar layers. The polarization fraction and polarization angle as measured by an instrument, however, do not necessary coincide with the intrinsic ones derived from models of surface emission. This is due to the effects of quantum electrodynamics in the highly magnetized vacuum around the star (the vacuum polarization) coupled with the rotation of the Stokes parameters in the plane perpendicular to the line of sight induced by the non-uniform magnetic field. Here we revisit the problem and present an efficient method for computing the observed polarization fraction and polarization angle in the case of radiation coming from the entire surface of a neutron star, accounting for both vacuum polarization and geometrical effects due to the extended emitting region. Our approach is fairly general and is illustrated in the case of blackbody emission from a neutron star with either a dipolar or a (globally) twisted magnetic field.