Disentangling the stellar atmosphere and the focused wind in different accretion states of Cygnus X-1

Abstract

Context. In high-mass X-ray binaries (HMXBs), the compact object accretes the strong stellar wind of an O-B supergiant companion star. X-ray flux variations alter the stellar wind's ionization state and optical line profiles, which are important in the determination of the orbital parameters of the system. Aims. We analyzed the state-dependent variability of the line profiles by separating the components coming from the star's atmosphere and the accreted stream of matter located between the star and the accretion disk (i.e. the focused wind). We then determined the radial velocities and the intensities of the absorption and emission lines with respect to the continuum. Methods. We performed optical high-resolution spectroscopy of the HMXB Cyg X-1 in the hard and soft-intermediate X-ray spectral states, respectively from 2022 and 2023, over multiple orbital phases. We then applied the method of Fourier disentangling to combine the spectra and isolate the stellar atmosphere and focused wind components. Results. We observe P-Cygni profiles of the Hα line in the stellar atmosphere and a wide emission from the focused wind, indicating an outflowing material. While He I λ6678 is in absorption in the stellar atmosphere and not detected in the focused wind, we see a broad emission feature of He II λ4686 in the focused wind. Moreover, we identify an X-ray/optical anticorrelation traced by the strength of the line intensity. The intensity of the lines drops in the soft-intermediate spectral state and the lines are more absorbed at the inferior conjunction of the star. Conclusions. Our results confirm that the He II emission comes from the focused wind rather than the stellar atmosphere and is produced from the re-scattering of the resonance line due to high-density clumps in the focused wind. The X-ray/optical anticorrelation shows a stronger wind in the low-hard state and the lines are stronger at the inferior conjunction of the star.