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The primary orbit and the absorption lines of HDE 226868 (Cygnus X-1)
Ninkov, Z.; Walker, G. A. H.; Yang, S.
AA(British Columbia, University, Vancouver, Canada), AB(British Columbia, University, Vancouver, Canada), AC(British Columbia, University, Vancouver, Canada)
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 321, Oct. 1, 1987, p. 425-437. NSERC-supported research. (ApJ Homepage)
Publication Date:
NASA/STI Keywords:
Absorption Spectra, Cygnus Constellation, Stellar Orbits, X Ray Binaries, Black Holes (Astronomy), Helium, O Stars, Stellar Spectra
Bibliographic Code:


From Reticon spectra of about 1 A resolution taken between 1980 and 1984, the radial velocity curve of HDE 226868 is found to be characteristic of a single-line spectroscopic binary with K = 75.0 + or - 1.0 km/s and e = 0.0. Combining historical velocities from the literature with present data and applying a period-folding analysis, a period of 5.59964 + or - 0.00001 days is found. These values agree well with those published by Gies and Bolton (1982). The value of v sin i is estimated to be 94.3 + or - 5 km/s from CFHT Reticon spectra taken at 0.1 A resolution. Assuming that the rotation of the primary is synchronized to the orbital revolution of the secondary gives a primary to secondary mass ratio between 1.5 and 2.3. An absolute magnitude of -6.5 + or - 0.2 is derived from the equivalent width of H-gamma (1.5 + or - 0.1 A) and the calibration of Walker and Millward (1985), which is consistent with the spectral classification of O9.7 Iab. Assuming 20 solar masses as a reasonable estimate for the mass of the primary implies a mass of 10 + or - 1 solar masses for the secondary.

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