The long-term millimeter activity of active galactic nuclei
  We analyze the long-term evolution of the fluxes of six active galactic
nuclei (AGN) - 0923+392, 3C 111, 3C 273, 3C 345, 3C 454.3, and 3C 84 - in the
frequency range 80-267 GHz using archival calibration data of the IRAM Plateau
de Bure Interferometer. Our dataset spans a long timeline of ~14 years with
974-3027 flux measurements per source. We find strong (factors ~2-8) flux
variability on timescales of years for all sources. The flux density
distributions of five out of six sources show clear signatures of bi- or even
multimodality. Our sources show mostly steep (alpha~0.5-1), variable spectral
indices that indicate outflow dominated emission; the variability is most
probably due to optical depth variations. The power spectra globally correspond
to red-noise spectra with five sources being located between the cases of white
and flicker noise and one source (3C 111) being closer to the case of random
walk noise. For three sources the low-frequency ends of their power spectra
appear to be upscaled in spectral power by factors ~2-3 with respect to the
overall powerlaws. In two sources, 3C 454.3 and 3C 84, the 1.3-mm emission
preceeds the 3-mm emission by ~55 and ~300 days, respectively, probably due to
(combinations of) optical depth and emission region geometry effects. We
conclude that the source emission cannot be described by uniform stochastic
emission processes; instead, a distinction of "quiescent" and (maybe multiple)
"flare" states of the source emission appears to be necessary.
