The classification of X-ray binaries is somewhat ambiguous. Some authors consider X-ray binaries to be any kind of interacting close binary with a compact degenerate object - that is, a white dwarf, a neutron star, or a black hole. A more specific definition is that X-ray binaries are only those interacting close binary systems which contain a neutron star or a black hole. In this chapter we shall restrict ourselves to the latter definition; interacting close binaries with a white dwarf are usually called cataclysmic variables which are described in Chapter 5 of this book.

The main (empirical) difference between the cataclysmic variable and the X-ray binaries as defined above is the X-ray luminosity: whereas X-ray binaries have X-ray luminosities of 1O∧35 - 10 ∧38 erg s which corresponds to 25 to 25000 times the total solar luminosity, cataclysmic variables have Lx ≦ 1034 erg s. Hence, X-ray binaries are discovered on the basis of their strong X-ray emission. The basic model of X-ray binaries is a close binary system with a ‘normal’ star (main sequence or giant, in exceptional cases a degenerate star too) filling its Roche lobe and transferring matter to the compact object, a neutron star or a black hole. Such a system is called a ‘semi-detached’ system. Due to the orbital angular momentum the matter cannot directly fall onto the compact object, and it forms an accretion disc around the latter (see also Section 5.4). Due to internal friction in the accretion disc (also called viscosity) the matter spirals inward until it eventually falls onto the compact object.