标题：Chandra X-ray spectroscopy of focused wind in the Cygnus X-1 system - II. The non-dip spectrum in the low/hard state – modulations with orbital phase
其他标题：II. The non-dip spectrum in the low/hard state – modulations with
orbital phase
摘要：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 using 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.
