Mechanisms for Outflow in AGNs

Abstract

In active galactic nuclei (AGNs), radiation pressure is important for the dynamics of the line-emitting clouds. In SS433, the steady speed of the jets (~ 0.26 c) suggests that radiative acceleration may be responsible; but there are difficulties with this interpretation, and it is perhaps unlikely that analogous effects occur in AGNs. In the pair-dominated plasmas expected in AGNs, radiation pressure is competitive with gravity even for luminosities below the normal Eddington limit. For realistic geometries, radiation-driven pair-dominated winds may arise. The most dramatic outflow from AGNs, however, is manifested by the relativistic jets observed predominantly in the radio band. Although the plasma in these jets may be predominantly electron-positron pairs, the outflow is probably driven electromagnetically rather than radiatively: most of the energy may initially be Poynting flux, which is converted into fast particles in the “blobs” whose apparently superluminal motion is revealed by VLBI. Finally, the possibility is raised that sporadic outflow may result from tidal disruption of stars near a massive black hole: such a process may have observable conseguences in our Galactic Centre.