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

TL;DR

This study uses Fourier Disentangling to separate stellar atmosphere and focused wind contributions in Cygnus X-1, revealing state-dependent spectral variability and wind properties in different X-ray states.

Contribution

It introduces a method to disentangle stellar and wind spectral features in HMXBs, providing new insights into wind behavior across accretion states.

Findings

Detection of H-alpha and He II in both wind and photosphere

Observation of an X-ray/optical anticorrelation in line intensities

Evidence of a stronger wind in the low-hard state

Abstract

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 key in the determination of the orbital parameters of the system. Using the method of Fourier Disentangling, we decomposed the spectral contributions from the stellar atmosphere close to the photosphere and the accreted stream of matter (i.e. the focused wind). High-resolution optical spectroscopy of Cyg X-1 in its hard and soft-intermediate X-ray states revealed state-dependent variability in the line profiles. In both states, we detect H-alpha and He II in both the focused wind and the stellar photosphere, whereas He I is not detected in the focused wind. Additionally, we observe an X-ray/optical anticorrelation, where the lines' intensity decreases in the soft-intermediate state and the lines are more absorbed at the inferior conjunction of the star. These results suggest a stronger wind in the low-hard state and the presence of high-density clumps in the line of sight at the conjunction.