Polarized thermal emission from X-ray Dim Isolated Neutron Stars: the case of RX J1856.5-3754

TL;DR

This paper investigates the polarized thermal emission from the neutron star RX J1856.5-3754, showing how polarization measurements can reveal surface composition and aid future X-ray polarimetry studies.

Contribution

It models polarization signatures from different surface states of neutron stars, highlighting how polarization data can distinguish between atmospheric and condensed surfaces.

Findings

Polarization measurements can determine surface composition.

Vacuum polarization effects influence observed polarization.

Optical and X-ray polarization data are complementary.

Abstract

The observed polarization properties of thermal radiation from isolated, cooling neutron stars depend on both the emission processes at the surface and the effects of the magnetized vacuum which surrounds the star. Here we investigate the polarized thermal emission from X-ray Dim Isolated Neutron Stars, taking RX J1856.5-3754 as a representative case. The physical conditions of the star outermost layers in these sources is still debated, and so we consider emission from a magnetized atmosphere and a condensed surface, accounting for the effects of vacuum polarization as the radiation propagates in the star magnetosphere. We have found that, for a significant range of viewing geometries, measurement of the phase-averaged polarization fraction and phase-averaged polarization angle at both optical and X-ray wavelengths allow us to determine whether this neutron star has an atmosphere or a condensed surface. Our results may therefore be relevant in view of future developments of soft X-ray polarimeters.