Ultraviolet Observations of the X-Ray Photoionized Wind of Cygnus X-1 during X-Ray Soft/High State
Abstract
High-resolution ultraviolet observations of Cygnus X-1 were obtained at two epochs roughly 1 year apart using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. Orbital phase ranges around phiorb = 0 and 0.5 were covered at each epoch, and the spectra show P Cygni line features from high (N V, C IV, Si IV) and absorption lines from low (Si II, C II) ionization state material. The high-ionization P Cygni profiles show a marked dependence on orbital phase; strong, broad absorption components when the X-ray source is behind the companion star and noticeably weaker absorption when the X-ray source is between us and the companion star. We fit the P Cygni profiles using the Sobolev with exact integration method applied to a spherically symmetric stellar wind subject to X-ray photoionization from the black hole. Of the wind-formed lines, the Si IV doublet provides the most reliable estimates of the parameters of the wind and X-ray illumination. The velocity v increases with radius r (normalized to the stellar radius) according to v = v∞(1 - rstar/r)β, with β ≈ 0.75 and v∞ ≈ 1420 km s-1. Our fit implies a ratio of X-ray luminosity (in units of 1038 erg s-1) to wind mass-loss rate (in units of 10-6 M⊙ yr-1) of LX,38/dot M-6 ≈ 0.033, measured at dot M-6 = 4.8. Our models determine parameters that may be used to estimate the accretion rate onto the black hole and independently predict the X-ray luminosity. Our predicted LX matches that determined by contemporaneous RXTE ASM remarkably well, but is a factor of 3 lower than the rate according to Bondi-Hoyle-Littleton spherical wind accretion. We suggest that some of the energy of accretion may go into powering a jet.
Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. These observations are associated with programs GO-9646 and GO-9840.- Publication:
-
The Astrophysical Journal
- Pub Date:
- May 2008
- DOI:
- 10.1086/586692
- arXiv:
- arXiv:0710.3499
- Bibcode:
- 2008ApJ...678.1248V
- Keywords:
-
- black hole physics;
- ultraviolet: stars;
- Astrophysics
- E-Print:
- 34 pages, 21 figures, 4 tables, accepted for publication in ApJ