Effects of thermal plasma on self-absorbed synchrotron sources in active galactic nuclei

We review the observable effects of a thermal background plasma in a self-absorbed synchrotron source, in the context of a model for the central engine of an active galactic nucleus (AGN). Considering the effects of free-free absorption and emission, Thomson and Compton scattering, and spatial stratification, we find that the observations set an upper limit on the thermal electron scattering optical depth in the central synchrotron-emitting region of an AGN. The upper limit, τ_max_ ~ 1, results mainly from the apparent absence of induced Compton scattering and inverse thermal Comptonization effects. The low value of τ_max_ poses some problems for nonthermal models of the AGN continuum that can be partly resolved by assuming a thin disk or layer-like geometry for the source, with (h/R) <~ 10^-2^. A likely site for the synchrotron-producing region seems to be the surface of an accretion disk or torus.