Observation and Interpretation of the Cygnus X -1 System.

The results of a long term monitoring program on the massive X-ray binary Cygnus X-1 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 with an orbital period of 5.59977 (+OR-) 0.00001 days. A P/P (DBLTURN) 10('-5) day('-1) was found from analysis of all available velocity measures. The determined K amplitude (75.0 (+OR-) 1 km/sec) and vsini (94.3 km/sec) allow the ratio m(,p)/m(,x) to be deter- mined as (DBLTURN) 2.0. Measurement of the interstellar lines reveals that the interstellar line strength per unit E(B-V) is lower than in any other direction in the sky. The mass loss rate for HDE 226868 (the primary of Cygnus X -1) was determined using the velocity excitation gradient as 5.7 (+OR-) 2 x 10('-6) M(,o)/year. The He II (lamda)4686 and H(alpha) lines are found in emission. The radial velocity curve of the corrected He II (lamda)4686 line is found to have small scatter from a smooth fit ((+OR-)10 km/sec) with no significant eccentri - city. The He II emission equivalent width shows strong modulation (30%) over the 5.6 day orbital period. The H(gamma) and H(beta) lines show a 20% variation on the 294 day X-ray period in the sense of largest equivalent widths at X-ray minimum (0 phase). The origin of the He II (lamda)4686 emission is explained by using a model in which a stellar wind enhanced in the direction of the secondary results in enhanced He II production as a result of X-ray heating and ionization. X-ray observations were made with EXOSAT and absorption dips were seen near orbital phase zero. Two scale lengths of dips were found of 10('8) and 10('11) cm with column density for the larger of 2.0 x 10('23) cm('-2). These values agree with theoretical predictions for the sizes of inhomogeneities in stellar winds.