An Orbital Model for the X-Ray Transient CEN X-4

Spectroscopic observations of the X-ray transient Cen X-4 in its optically faint state reveal the absorption spectrum of a late-type star. The velocity of this star is clearly modulated with the photometric period of about 15 hr (0.629 days) found by Ilovaisky and Chevalier (1986). Phasing of three epochs of absorption-line velocities gives a revised orbital period of 0.628935 + or - 0.000051 days. The orbital parameters show that the system contains two low-mass stellar objects, the observed late-type star and a collapsed companion (presumably a neutron star), which is the X-ray source. However, unlike most low-mass X-ray binaries, the emission lines (principally H, He I, He II, and Ca II) must originate near the late-type star, as their velocities vary approximately in phase with the late-type star rather than with the neutron star and its accretion disk. The exact phasing of the emission velocity differs between observing runs and may depend on the system's brightness. The light curve shows two minima during one orbital cycle, with the primary minimum occurring at spectroscopic phase 0.25, when the late-type star is most distant from the observer.