Origin and Evolution of X-Ray Binaries in Globular Clusters

Abstract

The relative numbers of low luminosity to high luminosity X-ray sources in globular clusters may be understood if the neutron stars in the latter were formed from high mass transfer onto the white dwarfs in a fraction of the former. That fraction is related to the ratio of cluster giants to main sequence stars, or about 1%. Optical observations of the fields of low luminosity sources in several globulars, particularly NGC 5824, as well as the optical candidate for the high luminosity source in M15 are consistent with this picture. Subsequent evolution of these compact X-ray binaries in the high density cluster cores in which they are formed by tidal capture will lead to the formation of hierachical triple systems (but probably not higher order systems). The fact that such systems may have been detected via long X-ray periods in a number of X-ray burst sources in and out of globular clusters argues that the burst sources apparently outside globular clusters and in the galactic bulge may be the remnants of disrupted globular clusters. Such a model may be required for the origin of the peculiarly high luminosity X-ray burster GX17+2 (one of several bursters previously suggested to be a globular cluster remnant) which has been recently found to exhibit the QPO phenomena and which may be a triple system with inner period 1.3 hours and outer period 6.4 days.