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 that 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 H- and He-like resonance lines from Ne, Na, Mg, and Si. We calculate a flux-weighted mean redshift of ≃100 km s-1 and a flux-weighted mean velocity width of ≃800 km s-1 (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: a rather large range of temperatures is implied, and it is unlikely that a narrow range of ionization parameters can account for the lines observed. 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.