High resolution optical and infrared spectroscopic observations of Cir X-1

We present new optical and infrared (IR) observations of Cir X-1 taken near apastron. Both sets of spectra show asymmetric emission lines. Archival optical observations show that an asymmetric H-alpha emission line has been in evidence for the past twenty years, although the shape of the line has changed significantly. We present an eccentric (e ~ 0.7-0.9) low mass binary model, where the system consists of a neutron star orbiting around a (sub-)giant companion star of 3-5 solar masses. We suggest that the broad components of the emission lines arise in a high-velocity, optically thick flow near the neutron star; while the narrow components of the optical and the IR lines arise near the companion star and a heated ejecta-shell surrounding the binary respectively. In this model, the velocity of the narrow component reflects the space velocity of the binary; the implied radial velocity (+430 km/s after correcting for Galactic rotation) is the highest velocity known for an X-ray binary