X-ray binaries are responsible for the bulk of the X-ray emission of our own galaxy. A lot has been learned about these bright X-ray sources since the beginning of X-ray astronomy, but significant questions are still open. These questions are related to the origin and evolution of these sources, and to how their properties depend on those of the parent stellar population. The discovery of several populations of X-ray binaries in external galaxies with Chandra, and to a lesser extent with XMM-Newton, gives us tools to look at these sources in a new way. Not only can we reconsider long-standing questions of galactic studies, such as the origin of low-mass X-ray binaries, but also we can look at the entire gamut of X-ray binary properties in a range of environments, from actively star-forming galaxies to older stellar systems. These observations have led to the discovery of several ultraluminous X-ray sources, thereby introducing new interesting possibilities for our understanding of X-ray binaries and possibly opening new paths to the discovery of the elusive intermediate-mass black holes.
5.1 Introduction and chapter outline
X-ray astronomy began with the unexpected discovery of a very luminous source, Sco X-1 (Giacconi et al., 1962), the first galactic X-ray binary (XRB) ever to be observed. XRBs, the most common luminous X-ray sources in the Milky Way, are binary systems composed of an evolved stellar remnant (neutron star [NS], black hole [BH], or white dwarf [WD]), and a stellar companion (for reviews on XRBs, see Lewin et al., 1995; Lewin and van der Klis, 2006).