Beaming of polarized radiation in subcritical X-ray pulsars

TL;DR

This study models the beaming and polarization of X-ray radiation in subcritical pulsars, revealing how resonant Compton scattering influences observed light curves and polarization signals.

Contribution

It provides a numerical analysis of polarization-dependent radiative transfer near neutron stars, incorporating vacuum polarization and relativistic effects.

Findings

Resonant Compton scattering significantly alters beaming patterns.

Angular redistribution near the cyclotron resonance reduces pulse modulation.

The model explains the suppressed pulsed fraction at certain energies.

Abstract

Radiation of X-ray pulsars is powered by accretion on the neutron star surface from a binary companion under the influence of a strong magnetic field. We study beaming of this radiation in the case of subcritical X-ray pulsars, where it is formed in the accretion channel close to the neutron star surface. We solve equations of the hydrodynamics and radiative transfer of two coupled polarization modes in the accretion channel numerically, taking into account resonant Compton scattering and vacuum polarization. The beaming patterns are obtained for different accretion rates, photon energies and polarizations, and for different models of the neutron star surface radiation. The calculated beaming patterns are converted into light curves for both the intensity and polarization, taking into account the effects of General Relativity. These beaming patterns and light curves are found to be strongly affected by the resonant Compton scattering for photon energies comparable with the electron cyclotron energy. In particular, the angular redistribution of radiation near the cyclotron resonance may reduce the light-curve modulation amplitude, which is consistent with observational indications of a suppressed pulsed fraction at these energies.