Monte Carlo Simulations of Polarized Radiative Transfer in Neutron Star Atmospheres

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 (${\omega }_{\mathrm{B}}$) frequencies once $\omega /{\omega }_{\mathrm{B}}\lesssim 0.01$. 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.

25 pages, 12 figures, accepted for publication in The Astrophysical Journal

References in corpus (11)

• Atmosphere Models of Magnetized Neutron Stars: QED Effects, Radiation Spectra, and Polarization Signals
• Polarized x-rays from a magnetar
• X-ray spectra and polarization from magnetar candidates
• Models of magnetized neutron star atmospheres: thin atmospheres and partially ionized hydrogen atmospheres with vacuum polarization
• A strong X-ray polarization signal from the magnetar 1RXS J170849.0-400910
• IXPE Detection of Polarized X-rays from Magnetars and Photon Mode Conversion at QED Vacuum Resonance
• Simultaneous magnetic polar cap heating during a flaring episode from the magnetar 1RXS J170849.0-400910
• Polarized Radiation Transfer in Neutron Star Surface Layers
• Intensity and Polarization Characteristics of Extended Neutron Star Surface Regions
• High-Energy Photon Opacity in the Twisted Magnetospheres of Magnetars
• Measuring the Non-Axially-Symmetric Surface Temperature Distribution of the Central Compact Object in Puppis A

Cited by in corpus (2)

• Vacuum birefringence and the polarized X-ray emission from a radio magnetar
• Pulsed, Polarized X-ray Emission from Neutron Star Surfaces: the Effects of Vacuum Birefringence in the Magnetosphere