Orbital elements and an analysis of models for HDE 226868 = Cygnus X-1

Abstract

Radial velocities from 21 new high-dispersion spectrograms of HDE 226868 are presented. These are combined with previously published data to calculate a ''definitive'' set of orbital elements for the binary system. In particular, archival data are used to obtain a precise period. The ellipsoidal light curve is analyzed using both a Roche model and an ellipsoidal model, and the results are compared with work by Hutchings. Information from the absorption-line and emission-line velocity curves and the light curve is combined to give estimates for the orbital inclination and the component masses. The possible errors in the analysis are discussed and are shown to be negligible. A qualitative model for the mass transfer is proposed that explains the intensity and velocity variations of the optical emission lines and the variations in the X-ray intensity: including the low-energy X-ray absorption events sometimes seen near superior conjunction of the secondary. Tests of this model are also proposed. Finally, the observations are used to test various models that have been proposed for the system. The observations rule out low mass and rotating degenerate dwarf secondaries and present difficulties for the triple star model. The magnetic reconnection model is not ruled out by the observations.more » Models in which the secondary is a black hole are consistent with all available observations. (AIP)« less