Revealing the Focused Companion Wind in Cygnus X-1 with Chandra

We have analyzed a Chandra/HETGS spectrum of the Galactic black hole Cygnus X-1, obtained at a source flux which is approximately twice that commonly observed in its persistent low-intensity, spectrally-hard state. We find a myriad of absorption lines in the spectrum, including Ly-α lines and helium-like resonance lines from Ne, Na, Mg, and Si. We calculate a flux-weighted mean red-shift of' 100 km/s and a flux-weighted mean velocity width of ' 800 km/s (FWHM) for lines from these elements. We also detect a number of transitions from Fe XVIII– XXIV and Ni XIX–XX in absorption; however, the identification of these lines is less certain and a greater range of shifts and breadth is measured. Our observation occurred at a binary phase of φ ' 0.76; the lines observed are consistent with absorption in an ionized region of the supergiant O9.7 Iab companion wind. The spectrum is extremely complicated in that a range of temperatures and densities are implied. Prior Chandra/HETGS spectra of Cygnus X-1 were obtained in a similar transition state (at φ ' 0.93) and in the low/hard state (at φ ' 0.84). Considered together, these spectra provide evidence for a companion wind that is focused as it flows onto the black hole primary in this system.