Accretion Discs in AGN Context: Hints Toward Non-Standard Discs?

Summary

Abstract

The structure and emission properties of accretion discs are briefly reviewed, with special attention given to geometrically thin “standard” accretion discs. Different approximations used to compute their emission spectrum are summarized, and it is shown that the bulk of the luminosity is emitted in the EUV range (the “UV bump”). However, according to recent optical, UV and X–ray monitoring campaigns of Seyfert galaxies, it appears that a large fraction of the observed optical–UV and even X–ray continuum is reprocessed by the disc from a primary source, probably of hard X–ray continuum, sitting near the black hole and heating the farther regions of the disc. A first consequence is that the disc must also contribute some fraction of the line emission. A second consequence is that standard accretion discs drawing their energy from viscosity and radiating it locally are ruled out in AGN, and another type of model must be built.

Generalities on accretion discs

For the reader who wishes to know more than just the basic properties that are recalled below, several reviews on accretion discs can be recommended: Pringle (1981), Frank, King & Raine (1992; revised version), Begelman (1985), Shields (1990), Treves, Maraschi & Abramovicz (1988).

If the matter accreted onto the black hole possesses angular momentum, it will settle in an accretion disc, in which matter is transported inward and angular momentum outward. This may be achieved through viscous friction, or through other stresses such as produced by a magnetic field.