Introduction: some historical remarks

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.

Clark (1975) suggested that the luminous cluster sources are binaries formed by capture from the remnants of massive stars. Fabian, Pringle and Rees (1975) specified that they are formed via tidal capture of neutron stars in close encounters with main-sequence stars. Sutantyo (1975) suggested direct collisions between giants and neutron stars as a formation mechanism.