Globular Cluster X-Ray Sources

Abstract

The discoveries of both steady and bursting X-ray sources in globular clusters have made them as interesting for high energy astrophysics and X-ray astronomy as they were for unlocking galactic structure in Shapley’ s day. Globular clusters are massive (˜ 105M⊙) spherical clusters of primarily low mass (⋞0.8 M⊙) evolved stars and may be the oldest systems in the Galaxy. Thus when several globular clusters were tentatively identified as containing X-ray sources (Giacconiet al. 1974), it was apparent that either dynamical or evolutionary processes must be occurring among the ancient stars in globular clusters to produce the relatively short-lived X-ray sources. The existence of these X-ray sources has prompted considerable discussion as to their origin, since they exist with ˜100 times the probability per unit mass in globular clusters as in the Galaxy as a whole (Katz 1975; Clark 1975). They may be yet another example of X-ray binaries (e. g., Clark 1975; Fabianet al. 1975) or be due to accretion onto a single collapsed object (black hole) at the center of the cluster (Bahcall and Ostrlker 1975; Silk and Arons 1975). At present both of these classes of models are possible and generally consistent with the data, though each has its particular difficulties. We shall review the observational data and then the models in an effort to point out the present balance of evidence for the binaryvs.black hole models as well as the most promising directions for future work. Given the possibly high incidence of X-ray bursters in globular clusters, our discussion will refer to both observations and recent models for bursters. While this review will primarily address the X-ray sources in globular clusters, it is obvious that their eventual understanding will contribute much to studies of globular clusters in general and will prompt future studies in particular areas.