Jets, black holes and disks in blazars
  The Fermi and Swift satellites, together with ground based Cherenkov
telescopes, has greatly improved our knowledge of blazars, namely Flat Spectrum
Radio Quasars and BL Lac objects, since all but the most powerful emit most of
their electro-magnetic output at gamma-ray energies, while the very powerful
blazars emit mostly in the hard X-ray region of the spectrum. Often they show
coordinated variability at different frequencies, suggesting that in these
cases the same population of electrons is at work, in a single zone of the jet.
The location of this region along the jet is a matter of debate. The jet power
correlates with the mass accretion rate, with jets existing at all values of
disk luminosities, measured in Eddington units, sampled so far. The most
powerful blazars show clear evidence of the emission from their disks, and this
has revived methods of finding the black hole mass and accretion rate by
modelling a disk spectrum to the data. Being so luminous, blazars can be
detected also at very high redshift, and therefore are a useful tool to explore
the far universe. One interesting line of research concerns how heavy are their
black holes at high redshifts. If we associate the presence of a relativistic
jet with a fastly spinning black hole, then we naively expect that the
accretion efficiency is larger than for non-spinning holes. As a consequence,
the black hole mass in jetted systems should grow at a slower rate. In turn,
this would imply that, at high redshifts, the heaviest black holes should be in
radio-quiet quasars. We instead have evidences of the opposite, challenging our
simple ideas of how a black hole grows.
