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

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.

18 pages, 10 figures, 3 tables, accepted for publication in MNRAS

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• Cygnus X-3 revealed as a Galactic ultraluminous X-ray source by IXPE
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• The INTEGRAL view on Black Hole X-ray Binaries
• Photoionization Emission Models for the Cyg X-3 X-ray Spectrum
• Ultrasoft state of microquasar Cygnus X-3: X-ray polarimetry reveals the geometry of astronomical puzzle
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• Probing the Stellar Wind of the Wolf-Rayet Star in IC 10 X-1
• The innermost jet in the hidden ultra-luminous X-ray source Cygnus X-3
• Wolf-Rayet -- compact object binaries as progenitors of binary compact objects
• Strong low-frequency radio flaring from Cygnus X-3 observed with LOFAR
• Cygnus X-3 as semi-hidden PeVatron
• Secrets behind the RXTE/ASM light curve of Cyg X-3
• Spectroscopic modelling of two high-mass X-ray binaries, Cyg X--3 and 4U 1538--522
• Extreme Transients in Gamma Rays
• Long-term study of the gamma-ray emission of Cygnus X-3 with MAGIC and Fermi-LAT
• Birthplaces of X-ray emission lines in Cygnus X-3