Globular cluster X-ray sources

Abstract

The earliest detections of luminous X-ray sources (LX ≳ 1036 erg s−1) in globular clusters were made with the Uhuru and OSO-7 Observatories (Giacconi et al. 1972, 1974; Clark, Markert & Li, 1975; Canizares & Neighbours, 1975). About 10% of the luminous X-ray sources in our Galaxy are found in globular clusters. This implies that the probability (per unit mass) of finding a luminous X-ray source in a globular cluster is about two to three orders of magnitude higher than of finding one in the rest of our Galaxy (Gursky 1973; Katz 1975). Clearly, the conditions in globular clusters are very special in that they must be very efficient breeding grounds for X-ray binaries. For reviews that reflect the ideas in the late seventies and early eighties, see Lewin (1980), Lewin & Joss (1983), van den Heuvel (1983) and Verbunt & Hut (1987). At that time there was no evidence for a substantial population of binaries in globular clusters; e.g., Gunn and Griffin (1979) did not find a single binary in a spectroscopic search for radial velocity variations of 111 bright stars in M3.