Polarized radiation from the spreading layer of the weakly magnetized neutron stars

TL;DR

This paper presents a theoretical model predicting that the polarization degree of X-ray emission from the spreading layer of weakly magnetized neutron stars is limited to 1.5%, aiding interpretation of recent polarization observations.

Contribution

The study introduces a model incorporating relativistic effects to estimate polarization from the spreading layer of neutron stars, providing new insights into X-ray polarization limits.

Findings

Polarization degree is limited to 1.5% regardless of geometry.

Relativistic aberration and gravitational light bending are significant factors.

Results assist in interpreting X-ray polarization observations.

Abstract

Observations show that the X-ray emission of the accreting weakly magnetized neutron stars is polarized. Here, we develop a theoretical model, where we assume the emission of the accreting neutron star coming from the spreading layer, the extension of the boundary between the disk and the neutron star surface onto the surface. We then calculate the Stokes parameters of the emission accounting for relativistic aberration and gravitational light bending in the Schwarzschild metric. We show that regardless of the geometry, for the spreading layer, we cannot expect the polarization degree to be higher than 1.5%. Our results have implications with regard to the understanding of the X-ray polarization from weakly magnetized neutron stars observed with the Imaging X-ray Polarimetry Explorer and the future enhanced X-ray Timing and Polarimetry mission.