7.3 The central engine
An object that fluctuates in brightness on a timescale Δt can have a radius no greater than R ∼ cΔt.
The point-like nature of AGNs and their rapid variability imply that the emitting region is smaller than the size of the Solar System.
The central engine of a typical AGN is believed to contain a supermassive black hole of mass ∼108M ⊙ and Schwarzschild radius ∼3 × 1011 m (2 AU).
Infalling material is thought to form an accretion disc around the black hole, converting gravitational energy into thermal energy and radiation. A typical AGN luminosity of 1038 W can be accounted for by an accretion rate of 0.2M⊙ per year.
The maximum luminosity of an accreting black hole is given by the Eddington limit, at which the gravitational force on the infalling material is balanced by the radiation pressure of the emitted radiation.
Jets are thought to be ejected perpendicular to the accretion disc.