Intensity and Polarization Characteristics of Extended Neutron Star Surface Regions

TL;DR

This paper models the intensity and polarization of X-ray emissions from extended neutron star surfaces, incorporating general relativistic effects, to aid in interpreting observational data and constraining stellar geometry.

Contribution

It extends previous localized surface models to include extended regions with varied magnetic fields, providing comprehensive polarization profiles for neutron star surfaces.

Findings

Phase-resolved polarization degrees of 10-60% are achievable.

Extended surface regions significantly influence observed polarization signatures.

The model supports interpretation of upcoming IXPE X-ray polarimetry data.

Abstract

The surfaces of neutron stars are sources of strongly polarized soft X rays due to the presence of strong magnetic fields. Radiative transfer mediated by electron scattering and free-free absorption is central to defining local surface anisotropy and polarization signatures. Scattering transport is strongly influenced by the complicated interplay between linear and circular polarizations. This complexity has been captured in a sophisticated magnetic Thomson scattering simulation we recently developed to model the outer layers of fully-ionized atmospheres in such compact objects, heretofore focusing on case studies of localized surface regions. Yet, the interpretation of observed intensity pulse profiles and their efficacy in constraining key neutron star geometry parameters is critically dependent upon adding up emission from extended surface regions. In this paper, intensity, anisotropy and polarization characteristics from such extended atmospheres, spanning considerable ranges of magnetic colatitudes, are determined using our transport simulation. These constitute a convolution of varied properties of Stokes parameter information at disparate surface locales with different magnetic field strengths and directions relative to the local zenith. Our analysis includes full general relativistic propagation of light from the surface to an observer at infinity. The array of pulse profiles for intensity and polarization presented highlights how powerful probes of stellar geometry are possible. Significant phase-resolved polarization degrees in the range of 10-60% are realized when summing over a variety of surface field directions. These results provide an important background for observations to be acquired by NASA's new IXPE X-ray polarimetry mission.