Chandra X-Ray Spectroscopy of the Focused Wind in the Cygnus X-1 System. II. The Nondip Spectrum in the Low/Hard State – Modulations with Orbital Phase

Kavli Affiliate: Norbert S. Schulz

| First 5 Authors: Ivica Miškovičová, Natalie Hell, Manfred Hanke, Michael A. Nowak, Katja Pottschmidt

| Summary:

The accretion onto the black hole in the system HDE 226868/Cygnus X-1 is
powered by the strong line driven stellar wind of the O-type donor star. We
study the X-ray properties of the stellar wind in the hard state of Cyg X-1 as
determined with data from the Chandra High Energy Transmission Gratings. Large
density and temperature inhomogeneities are present in the wind, with a
fraction of the wind consisting of clumps of matter with higher density and
lower temperature embedded in a photoionized gas. Absorption dips observed in
the light curve are believed to be caused by these clumps. This work
concentrates on the non-dip spectra as a function of orbital phase. The spectra
show lines of H-like and He-like ions of S, Si, Na, Mg, Al and highly ionized
Fe (Fe xvii-Fe xxiv). We measure velocity shifts, column densities, and thermal
broadening of the line series. The excellent quality of these five observations
allows us to investigate the orbital phase dependence of these parameters. We
show that the absorber is located close to the black hole. Doppler shifted
lines point at a complex wind structure in this region, while emission lines
seen in some observations are from a denser medium than the absorber. The
observed line profiles are phase dependent. Their shapes vary from pure,
symmetric absorption at the superior conjunction to P Cygni profiles at the
inferior conjunction of the black hole.

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