X-ray binaries in globular clusters

Abstract

It appears to be very unlikely that primordial binaries in globular clusters have evolved to produce high-luminosity X-ray sources like the four variable sources detected by the Uhuru and OSO-7 satellites. It is suggested that the cluster X-ray sources are binaries formed by capture from the remnants of massive single stars that exploded with sufficient isotropy to remain bound in the cluster. These remnants have lurked near the centers of the clusters where the high stellar densities have favored their capture of field stars which eventually turn the remnants on as X-ray sources when the captives enter the post-main-sequence expansion phase of their evolution. A case is made for the conclusion that there is approximately one potential binary X-ray source near the center of each of the globular clusters.