A model for 0921-63 - A second halo X-ray source

The optical counterpart of 0921-63 was spectroscopically observed with the CTIO 4 m telescope in February and November 1980 and February-March 1981, and the results are presented. With the assistance of published data, the orbital period was found to be 8.99 days. The He II and H emission are formed near the degenerate neutron star. The secondary, weakly seen in the spectrum, appears to be an F-G giant of about one solar mass. The system eclipses so that ultimately precise radii and orbital inclination can be determined. The galactic location, about 1.5 kpc below the plane, and the low mass, totalling about two solar masses, suggest that 0921-63 is a halo object. Although the rather long orbital period implies a separation of about 10 solar radii, the secondary is expected to fill its Roche lobe and transfer matter to the degenerate star.