The halo population X-ray source Cygnus X-2

Spectroscopic observations show the orbital period of V1341 Cyg (Cyg X-2) to be 9.843 days. Published photometric data, phased on this period, give a classical ellipsoidal (double-peaked) light curve with minima at the two conjunctions, implying the star is distorted to approximately its Roche lobe. The observed mass function, lack of X-ray eclipses, and analysis of the light curve restrict the masses to between 1.3 and 1.8 solar masses for the X-ray star and between 0.5 and 1.1 solar masses for the optical star. A new distance determination shows the system to be at about 8 kpc in the direction of galactic rotation (galactic longitude = 87 deg) and about 1.5 kpc above the plane. The large systemic velocity (-222 km/s), small mass, and possibly weak metals lines suggest the source is a halo object which, like the globular clusters, does not rotate with the galactic disk. Spectroscopic and photometric data show there may be variable X-ray heating on the optical star.