The constraining power of X-ray polarimetry: detailed structure of the intrabinary bow shock in Cygnus X-3

TL;DR

This study uses X-ray polarimetry data of Cygnus X-3 to model its intrabinary bow shock, revealing a cylindrical shock geometry with asymmetry and optical depth, advancing understanding of wind structures in high-mass X-ray binaries.

Contribution

The paper introduces an analytical model of polarized X-ray variability to constrain the geometry of the intrabinary shock in Cygnus X-3, highlighting a cylindrical shock structure with asymmetry.

Findings

Cylindrical bow shock best matches polarimetric data.

Asymmetry and optical depth are crucial for reproducing observations.

Shock geometry is similar to jet-wind interaction scenarios.

Abstract

Cygnus X-3 is the only known Galactic high-mass X-ray binary with a Wolf-Rayet companion. Recent X-ray polarimetry results with the Imaging X-ray Polarimetry Explorer have revealed it as a concealed ultraluminous X-ray source. It is also the first source where pronounced orbital variability of X-ray polarization has been detected -- notably with only one polarization maximum per orbit. Polarization caused by scattering of the source X-rays can only be orbitally variable if the scattering angles change throughout the orbit. Since this requires an asymmetrically distributed medium around the compact object, the observed variability traces the intrabinary structures. The single-peaked profile further imposes constraints on the possible geometry of the surrounding medium. Therefore, the X-ray polarization of Cygnus X-3 is the first opportunity to study the wind structures of high-mass X-ray binaries in detail. We aim to uncover the underlying geometry through analytical modeling of the polarized variability. Knowledge of these structures could be extended to other sources with similar wind-binary interactions. We study the variability caused by single scattering in the intrabinary bow shock, exploring both the optically thin and optically thick limits. We consider two geometries of the reflecting medium, the axisymmetric parabolic bow shock and the parabolic cylinder shock, and determine the geometry that best matches the X-ray polarimetric data. We find that the peculiar properties of the data can only be replicated with a cylindrical bow shock with asymmetry across the shock centerline and significant optical depth. This geometry is comparable to shocks formed by the jet-wind or outflow-wind interactions. The position angle of the orbital axis is slightly misaligned from the orientation of the radio jet in all our model fits.