X-ray Polarization from High Mass X-ray Binaries

TL;DR

This paper explores how X-ray polarization signatures in high-mass X-ray binaries depend on system geometry, orbital phase, and wind asymmetries, providing insights for future observational studies.

Contribution

It demonstrates that X-ray polarization from HMXBs has a distinct dependence on inclination, phase, and wind asymmetries, highlighting the potential of polarization measurements to probe system geometry.

Findings

Polarization varies with inclination and orbital phase.

Resonance line scattering can significantly increase polarization.

Wind asymmetries lead to polarization fractions up to tens of percent.

Abstract

X-ray astronomy allows study of objects which may be associated with compact objects, i.e. neutron stars or black holes, and also may contain strong magnetic fields. Such objects are categorically non-spherical, and likely non-circular when projected on the sky. Polarization allows study of such geometric effects, and X-ray polarimetry is likely to become feasible for a significant number of sources in the future. Potential targets for future X-ray polarization observations are the high-mass X-ray binaries (HMXBs), which consist of a compact object in orbit with an early type star. In this paper ws show that X-ray polarization from HMXBs has a distinct signature which depends on the source inclination and orbital phase. The presence of the X-ray source displaced from the star creates linear polarization even if the primary wind is spherically symmetric whenever the system is viewed away from conjunction. Direct X-rays dilute this polarization whenever the X-ray source is not eclipsed; at mid-eclipse the net polarization is expected to be small or zero if the wind is circularly symmetric around the line of centers. Resonance line scattering increases the scattering fraction, often by large factors, over the energy band spanned by resonance lines. Real winds are not expected to be spherically symmetric, or circularly symmetric around the line of centers, owing to the combined effects of the compact object gravity and ionization on the wind hydrodynamics. A sample calculation shows that this creates polarization fractions ranging up to tens of percent at mid-eclipse.