Introduction

Accretion means the accumulation of diffuse gas or matter onto some object under the influence of gravity. Accretion from the interstellar medium onto stationary and moving stars was the subject of a number of pioneering papers by Bondi, Lyttelton and Hoyle and, in the light of subsequent studies, these have proved to provide quite accurate predictions for the rate of accretion (Hoyle and Lyttleton, 1939; Bondi and Hoyle, 1944; Bondi, 1952). The subject was reinvigorated in the 1960s by the realisation that accretion of matter onto supermassive black holes is a remarkably effective means of accounting for the extreme properties of active galactic nuclei and, even more, by the discovery of intense X-ray sources associated with binary systems in our Galaxy. The discovery of these objects and the ensuing flourishing of theory ushered in a new epoch in high energy astrophysics. Accretion was also applied to binary systems involving white dwarf stars and these processes could account for the properties of cataclysmic variables.

Let us begin our analysis by deriving some of the simple relations which show how naturally accretion can account, in principle, for many of the key features of galactic X-ray sources and active galactic nuclei.

Accretion – general considerations

The efficiency of the accretion process

Consider the accretion of matter onto a star of mass M and radius R. If the matter falls onto the star in free-fall from infinity, it acquires kinetic energy as its gravitational potential energy becomes more negative.