Orbital modulation of X-ray emission lines in Cygnus X-3

TL;DR

This study investigates the orbital modulation of X-ray emission lines in Cygnus X-3, combining observations, photoionization models, and simulations to understand wind dynamics and system parameters.

Contribution

It introduces a comprehensive model of X-ray line formation in Cygnus X-3, integrating observational data with simulations to constrain system masses and geometry.

Findings

Emission lines show maximum blue-shift when the compact star is in front.

The wind's absorption component velocity remains around -900 km/s.

System mass and inclination constraints suggest either a neutron star or a black hole.

Abstract

We address the problem where the X-ray emission lines are formed and investigate orbital dynamics using Chandra HETG observations, photoionizing calculations and numerical wind-particle simulations.The observed Si XIV (6.185 A) and S XVI (4.733 A) line profiles at four orbital phases were fitted with P Cygni-type profiles consisting of an emission and a blue-shifted absorption component. In the models, the emission originates in the photoionized wind of the WR companion illuminated by a hybrid source: the X-ray radiation of the compact star and the photospheric EUV-radiation from the WR star. The emission component exhibits maximum blue-shift at phase 0.5 (when the compact star is in front), while the velocity of the absorption component is constant (around -900 km/s). The simulated FeXXVI Ly alpha line (1.78 A) from the wind is weak compared to the observed one. We suggest that it originates in the vicinity of the compact star, with a maximum blue shift at phase 0.25 (compact star approaching). By combining the mass function derived with that from the infrared HeI absorption (arising from the WR companion), we constrain the masses and inclination of the system. Both a neutron star at large inclination (over 60 degrees) and a black hole at small inclination are possible solutions.