Powerful extragalactic jets
  The Fermi, Swift and INTEGRAL satellites, together with ground based
(especially Cherenkov) telescopes made possible a great progress in our
understanding of relativistic jets. We can now start to attack the difficult
questions of jet formation, collimation and content. We can also used them as
probes to quantify the amount of IR and optical background radiation, and the
amount of the cosmic magnetic field. Since they are the most powerful steady
sources of the Universe, we can study them also at large redshifts, and this is
a very fruitful field of research. To this aim, I will emphasize the importance
of high energy X-rays, where very powerful blazars are predicted to emit most
of their electromagnetic power. For them, the emission of the underlying
accretion disk becomes unhidden by the non-thermal jet radiation, allowing to
estimate the black hole mass and the accretion rate. In turn, this highlights
the connection between the disk and the jet. Since the highest power blazars
could have their emission peak in the ~MeV band, hard X-ray instruments could
be more appropriate than the Fermi/LAT to detect them.
