Magnetic flux paradigm for radio-loudness of AGN
  We argue that the magnetic flux threading the black hole, rather than black
hole spin or Eddington ratio, is the dominant factor in launching powerful jets
and thus determining the radio loudness of active galactic nuclei (AGN). Most
AGN are radio quiet because the thin accretion disks that feed them are
inefficient in depositing magnetic flux close to the black hole. Flux
accumulation is more likely to occur during a hot accretion (or thick disk)
phase, and we argue that radio-loud quasars and strong emission-line radio
galaxies occur only when a massive, cold accretion event follows an episode of
hot accretion. Such an event might be triggered by the merger of a giant
elliptical galaxy with a disk galaxy.
  This picture supports the idea that flux accumulation can lead to the
formation of a so-called magnetically choked accretion flow (MCAF). The large
observed range in radio loudness reflects not only the magnitude of the flux
pressed against the black hole, but also the decrease in UV flux from the disk,
due to its disruption by the "magnetosphere" associated with the accumulated
flux. While the strongest jets result from the secular accumulation of flux,
moderate jet activity can also be triggered by fluctuations in the magnetic
flux deposited by turbulent, hot inner regions of otherwise thin accretion
disks, or by the dissipation of turbulent fields in accretion disk coronae.
These processes could be responsible for jet production in Seyferts and
low-luminosity AGN, as well as jets associated with X-ray binaries.
