Monte Carlo Simulations of Polarized Radiative Transfer in Neutron Star Atmospheres

TL;DR

This paper introduces a Monte Carlo simulation method for modeling polarized X-ray emission from neutron star atmospheres across various magnetic field strengths, providing new analytical fits and insights into polarization behavior.

Contribution

The paper develops the MAGTHOMSCATT Monte Carlo code to simulate polarization in neutron star atmospheres, including analytical fits and a refined injection protocol for magnetar regimes.

Findings

Reproduced non-magnetic Thomson scattering solutions.

Provided analytical fits for intensity and polarization angular dependence.

Discovered invariance of pulse profiles at low photon to cyclotron frequency ratios.

Abstract

Soft X-ray emission from neutron stars affords powerful diagnostic tools for uncovering their surface and interior properties, as well as their geometric configurations. In the atmospheres of neutron stars, the presence of magnetic fields alters the photon-electron scattering cross sections, resulting in non-trivial angular dependence of intensity and polarization of the emergent signals. This paper presents recent developments of our Monte Carlo simulation, MAGTHOMSCATT, which tracks the complex electric field vector for each photon during its transport. Our analysis encompasses the anisotropy and polarization characteristics of X-ray emission for field strengths ranging from non-magnetic to extremely magnetized regimes that are germane to magnetars. In the very low field domain, we reproduced the numerical solution to the radiative transfer equation for non-magnetic Thomson scattering, and provided analytical fits for the angular dependence of the intensity and the polarization degree. These fits can be useful for studies of millisecond pulsars and magnetic white dwarfs. By implementing a refined injection protocol, we show that, in the magnetar regime, the simulated intensity and polarization pulse profiles of emission from extended surface regions becomes invariant with respect to the ratio of photon ($\omega$) and electron cyclotron ($\omega_{\rm B}$) frequencies once $\omega / \omega_{\rm B} \lesssim 0.01$. This circumvents the need for simulations pertinent to really high magnetic field strengths, which are inherently slower. Our approach will be employed elsewhere to model observational data to constrain neutron star geometric parameters and properties of emitting hot spots on their surfaces.