Modeling surface radiation of rotating neutron stars with Monk-NS

TL;DR

This paper introduces Monk-NS, a relativistic Monte Carlo radiative transfer code for modeling the electromagnetic emission of rotating neutron stars, accounting for scattering media and complex surface features, validated through benchmarking and applied to low-mass X-ray binaries.

Contribution

The paper presents Monk-NS, a novel specialized code for realistic modeling of neutron star surface emission, including scattering effects and complex hotspot geometries.

Findings

Polarisation degree depends strongly on observer inclination.

Complex hotspot shapes affect polarisation properties.

Modeling helps distinguish neutron star emission scenarios.

Abstract

Neutron stars serve as unique laboratories for studying ultra-dense nuclear matter. The equation of state of neutron star matter can be effectively constrained by their masses and radii. Particular attention has been paid to rapidly rotating neutron stars, where strong relativistic effects leave imprints on their electromagnetic emission. To model the emission of rotating neutron stars in more realistic situations, especially when their surface emission is further re-processed by a scattering medium, we develop Monk-NS, a customized version of the general relativistic Monte-Carlo radiative transfer code Monk. We validate the code through a series of benchmarking tests, including computing the energy spectrum, pulse profile, and polarisation of rotating neutron stars, and comparing the results with those of the established codes in the X-ray timing community, yielding consistent outcomes. As an example to demonstrate Monk-NS's capabilities, we apply it to investigate various models proposed to explain the low pulsation amplitude of neutron star low-mass X-ray binaries. Our findings indicate that the dependence of the X-ray polarisation degree on the observer's inclination can serve as a key factor in distinguishing these models. We also find that complex hotspot morphologies yield polarisation properties different from those of circular hotspots.