Introduction

In the 1995 X-ray Binaries book edited by Lewin, van Paradijs and van den Heuvel, the chapter on Normal galaxies and their X-ray binary populations (Fabbiano 1995) began with the claim that “X-ray binaries are an important component of the X-ray emission of galaxies. Therefore the knowledge gathered from the study of Galactic X-ray sources can be used to interpret X-ray observations of external galaxies. Conversely, observations of external galaxies can provide us with uniform samples of X-ray binaries, in a variety of different environments.” This statement was based mostly on the Einstein Observatory survey of normal galaxies (e.g., Fabbiano 1989; Fabbiano, Kim & Trinchieri 1992). Those results have been borne out by later work, yet at the time the claim took a certain leap of faith. Now, nearly a decade later, the sensitive sub-arcsecond spectrally resolved images of galaxies from Chandra (Weisskopf et al. 2000), complemented by the XMM-Newton (Jansen et al. 2001) data for the nearest galaxies (angular resolution of XMM-Newton is ∼15″), have made strikingly true what was then largely just wishful anticipation.

While a substantial body of ROSAT and ASCA observations exists, which was not included in the 1995 chapter, the revolutionary quality of the Chandra (and to a more limited degree of XMM-Newton) data is such that the present review will be based on these most recent results.