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

¹ Dr. Karl Remeis-Sternwarte and Erlangen Centre for Astroparticle Physics, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
e-mail: joern.wilms@sternwarte.uni-erlangen.de
² Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, CA 94550, USA
³ MIT Kavli Institute for Astrophysics and Space Research, NE80, 77 Mass. Ave., Cambridge, MA 02139, USA
⁴ CRESST, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁵ NASA Goddard Space Flight Center, Astrophysics Science Division, Code 661, Greenbelt, MD 20771, USA
⁶ AIT Austrian Institute of Technology GmbH, Donau-City-Str. 1, 1220 Vienna, Austria
⁷ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁸ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁰ Laboratoire AIM, UMR 7158, CEA/DSM-CNRS-Université Paris Diderot, IRFU/SAp, 91191 Gif-sur-Yvette, France
¹¹ Max-Planck Computing and Data Facility, Gießenbachstr. 2, 85748 Garching, Germany
¹² Department of Chemistry, Physics, and Astronomy, Georgia College & State University, Milledgeville, GA 31061, USA
¹³ Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley, CA 94720-7450, USA
¹⁴ Harvard John A. Paulson School of Engineering and Applied Sciences, and Harvard-Smithsonian Center for Astrophysics, 60 Garden Street MS-6, Cambridge, MA 02138, USA

Received: 14 August 2013
Accepted: 1 April 2016

Abstract

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.