Accretion Discs in Astrophysics

Abstract

If we put a particle in a circular orbit around a central gravitating body, it will stay in that orbit. If we then extract energy and angular momentum from the particle we may allow it to spiral slowly inwards. The amount of energy that can be extracted by such a process is equal to the binding energy of the innermost accessible orbit. For orbits around sufficiently compact objects a reasonable fraction of the particle's rest mass energy can be extracted. For example, of order 10 percent of the rest mass can be obtained from orbits around a neutron star and up to around 40 percent for orbits around a black hole. Thus, the accretion process can be an effi­ cient converter of rest mass to radiation. The problem is to set up the process that can extract the energy and angular momentum. If we consider a blob of gas in a circular orbit then we have more flexibility. In particular, if we can find a method of redistributing angular momentum among the gas particles in order to let some of them fall into the potential well, then we are in a position to extract the potential energy so released. The accretion disc provides just such a method. The efficiency with which energy is released and the ubiquity of angular momentum explains why accretion discs are popular in models for some of the most luminous objects-X-ray stars and quasars. However, accretion disc theory predates the discovery of both these, and it is to these initial developments that we now tum our attention.