Sub – parsec to Mega – parsec jet emission and power

Relativistic jets carry a significant fraction of the total energy budget of a radio source, rivaling the power that is extracted through accretion. A minor part of this bulk kinetic power is transformed to radiation, possibly through internal shocks if the plasma is accelerated, at the base of the jet, to a velocity which changes in time. In this way we can understand why some radiation is produced all along the jet even if most of it originates at a preferred location, and why the efficiency of conversion of bulk to random energy is small. The recent observations by Chandra of intense jet X–ray emission at large scales suggest that at least the “spine” of jets continues to be highly relativistics even up to hundreds of kiloparsecs away from the nucleus and give tight lower limits on the jet bulk kinetic power.