Neutron Stars and Black Holes in X-ray Binaries

Galactic accretion driven stellar X-ray sources can be divided into groups in different ways. An important division, which covers almost all known X-ray binaries, can be made according to the mass of the donor star: high-mass X-ray binaries and low-mass X-ray binaries. Another distinction (partially overlapping with the previous one) can be made on the basis of the nature of the accreting object: a strongly magnetized neutron star, a neutron star with a weak magnetic field, or a black hole. In this review I describe the properties of these different types of X-ray binaries, and discuss the mass determinations which are the basis for distinguishing accreting neutron stars from black holes. 1. Some Historical Background The connection between cosmic X-ray sources and compact stars is an old one: soon after the discovery of the first such source, Sco X-1 (Giacconi et al. 1963), it was proposed that these objects are young hot neutron stars, formed in recent supernovae, which cool through thermal radiation from their surfaces (Chiu 1964; Chiu & Salpeter 1964; Finzi 1964). However, the finite extent of the X-ray source associated with the Crab Nebula (Bowyer et al. 1964), and the non-Planckian shape of the X-ray spectra of this source (Clark 1965) and of Sco X-1 (Giacconi et al. 1965) showed that this was not, in general, a good model for X-ray sources. Accretion onto a compact star had meanwhile been suggested as a possible source of energy for quasars and X-ray sources (Salpeter 1964; Zel’dovich 1964; Zel’dovich & Guseinov 1964), and together with the peculiarities of the optical spectra of the counterparts of Sco X-1 (Sandage et al. 1966) and Cyg X-2 (Giacconi et al. 1967) this led to the idea that these sources are mass-exchanging binaries with a compact component (Shklovsky 1967; see