Plasma mechanisms for variability in active galactic nuclei

Abstract

We consider several physical processes, dominated by plasma effects, that might play roles in producing variability in Active Galactic Nuclei (AGN). One class of processes involves oscillations in the plasma itself. If accretion disks around supermassive black holes have coronae not too different from those of the Sun, then magnetoacoustic oscillations of coronal loops can yield fluctuations on timescales from days to years. Such coronal loops could also produce flares with substantial emission variations over the short times comparable to those associated with the recently observed broad-band microvariability. A second class of processes depends upon electromagnetic fields acting to modulate themselves; these modulational instabilities can produce filamentation and thence rapid variability. We also consider how a shift in emission and scattering mechanism (e.g., from Compton scattering to Raman scattering) can yield variability in the observed emission.