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-alpha 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 phi = 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 phi = 0.93) and in the low/hard state (at phi= 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. 
J.M. Miller (1), P. Wojdowski (1), N.S. Schulz (1), H.L. Marshall (1),   A.C. Fabian (2), R.A. Remillard (1), R. Wijnands (1...