Observation and interpretation of the Cygnus X-1 system
Creators: name: Ninkov, Zoran
Publisher: name: University of British Columbia
Year: 2010

description: The results of a long term monitoring program on the massive X-ray binary Cygnus X-1, whose constituents are believed to consist of a normal 0 star primary and a black hole companion, are presented. Spectra of this system were collected between 1980 and 1984 using a Reticon detector. The resulting absorption line radial velocity (RV) curve is characteristic of a single line spectroscopic binary. These velocities were combined with those available in the literature to determine an orbital period of 5.59977 ± 0.00001 days. A P/P ≃ 10⁻⁵ day⁻¹ was found from analysis of all available velocity measures. This change in the period is larger than that expected as a result of mass loss from the primary or from- models of the system in which large mass transfer rates occur between the components. A fit of the orbital motion of the primary to the RV curve gives a K = 75.0 ± 1 km/s and no significant eccentricity. The vsini of the primary was found, using the fourier transform technique, to be 94.3 km/sec. This is substantially smaller than the literature value of vsini = 140 km/sec. The value of the K and vsini allow the ratio mp/mx to be determined as ≃ 2.0 . The equivalent width of Hƴ allows the absolute magnitude of the primary to be estimated at -6.5 ± 0.2 . A comparison of the spectrum of the primary to those of an array of standards allows the spectral type to be given as between 09.5 and 09.7 I . This spectral type is consistent that the primary is a normal star of mass ≃ 20 M⊙. The mass of the secondary is therefore 10 ± 3 Mʘ. Measurement of the interstellar lines to obtain an independent E(B-V) reveals that the interstellar line strength per unit E(B-V) is lower than in any other direction in the sky. Stars for which velocity-excitation slopes and mass loss estimates, from UV line profile modeling and/or radio free-free emission measures, are available in the literature were collated. An empirical fit to this material allowed the mass loss rate for HDE 226868 (the primary of Cygnus X-1) to be estimated at 5.7 ± 2 x 10⁻⁶ M/year. The He II λ4686 and Hɑ lines are found in emission. After removal of the contribution to the line profile from the primary the radial velocity curve of the residual He II λ4686 line is found to have small scatter from a smooth fit ( ± 10 km/sec ) with no significant eccentricity. No sizeable variation in the K amplitude at different epochs was found contrary to a previous investigation and the origin of the emission is thus apparently fixed and stable. A phase lag of 130° is measured between the absorption and emission velocity curves and thus the simple interpretation of the emmision originating near the secondary can not be correct. The He II emission equivalent width, corrected for the underlying primary absorption, shows strong modulation (30%) over the 5.6 day orbital period. This variation is probably the result of the profile of the primary varying with which face of the star is directed towards the observer. During two separate observing sessions in 1982 the He II equivalent widths were found to be 40% and 15% larger than the mean of all other observations while still showing the same variation with orbital phase. Such a change has been seen once before and may be associated with transitions to the X-ray high state. The Hƴ and Hβ lines show a 20% variation on the 294 day X-ray period in the sense of largest equvalent widths at X-ray minimum ( 0 phase ). The Balmer lines are a composite of an absorption component from the primary and a weak emission component. This is best explained by variations in the outflow from the star, which is the source of both the emission component and the X-ray flux via accretion. Such variations may be the result of pulsation of the primary. The Hɑ line profile has been decomposed into three components; the absorption component from the primary, emission from a shell with an inner radius 1.4 times that of the primary, arid a component with properties similar to the He II λ4686 line. The great width of the Hɑ line, previously explained as being the result of rotation of the disc, is instead shown to be the result of superposition of these components. The origin of the He II λ4686 emission is explained by assuming that a stellar wind enhanced in the direction of the secondary is completely ionized within a volume surrounding the secondary. The He II between the edge of this volume and the surface of the primary is enhanced as a result of X-ray heating and ionization. Model profiles appear in reasonable agreement with high dispersion spectra. The obvious explanation for the orbital variation in the He II line is that X-ray heating of the side of the primary facing the secondary produces a change in the effective temperature. Calculation of the size of this effect reveals that it is too small to explain the changes observed. X-ray observations made with EXOSAT with excellent time resolution allowed timing of the X-ray absorption features seen near orbital phase zero. Simultaneous X-ray spectra allowed an estimate of their column density as 2.0 x 1023 cm⁻². Two scale lengths of dips were found of 10⁸ and 10¹¹ cm. These values are in good agreement with theoretical predictions for the sizes of inhomogeneties in high mass loss stellar winds. The location of the material producing the absorption dips was calculated as being ≃ 4-8 R⊙ from the X-ray source.