Gemini/GNIRS infrared spectroscopy of the Wolf-Rayet stellar wind in Cygnus X-3

TL;DR

This study uses infrared spectroscopy to analyze the Wolf-Rayet star in Cygnus X-3, revealing complex wind ionization and variability that challenge previous mass estimates and highlight the difficulty of deriving system parameters from spectra.

Contribution

First infrared spectral analysis of Cygnus X-3's Wolf-Rayet companion, proposing a classification and modeling its wind with non-LTE atmospheres considering ionization effects.

Findings

Orbital variations linked to wind ionization changes.

Inability to reproduce earlier mass function results.

Estimated properties of the Wolf-Rayet star from models.

Abstract

The microquasar Cygnus X-3 was observed several times with the Gemini North Infrared Spectrograph while the source was in the hard X-ray state. We describe the observed 1.0-2.4 $\mu$m spectra as arising from the stellar wind of the companion star and suggest its classification as a WN 4-6 Wolf-Rayet star. We attribute the orbital variations of the emission line profiles to the variations in the ionization structure of the stellar wind caused by the intense X-ray emission from the compact object. The strong variability observed in the line profiles will affect the mass function determination. We are unable to reproduce earlier results, from which the mass function for the Wolf-Rayet star was derived. Instead, we suggest that the system parameters are difficult to obtain from the infrared spectra. We find that the near-infrared continuum and the line spectra can be represented with non-LTE Wolf-Rayet atmosphere models if taking into account the effects arising from the peculiar ionization structure of the stellar wind in an approximative manner. From the representative models, we infer the properties of the Wolf-Rayet star and discuss possible mass ranges for the binary components.