The accretion paradigm states that most, and perhaps all, AGN are powered
by accretion onto a supermassive black hole (see, e.g.,
Blandford & Rees 1991).
Within this model
plays the most important role,
determining the emission properties,
and therefore the appearance, of the central source. Objects with high
accretion rate (
0.1) appear as an
``optical'' quasar
(of course, equally bright, if not brighter, in X-rays as well), while
low sub-Eddington accretion (
10-2) produces
a weak
``radio'' core with substantially less optical emission. A zero
accretion rate produces a ``dead'' quasar - a black hole detectable only
through its gravitational influence on the galactic nucleus. For a given
level, the black hole mass
determines mainly the luminosity scaling.