Abstract:
Relativistic jets in Active Galactic Nuclei transport energy from the supermassive black hole at the center of their host galaxy to distances that can exceed ten galactic radii. Not easy to measure, the power carried by these jets is a crucial quantity for understanding the physics of jet formation and the impact on the surrounding medium/environment. Pioneering attempts made in the past hinted to an approximate equality between jet power and disk luminosity, although with large uncertainties in the method and dispersion in the results. Now, with the advent of the Fermi satellite, there are hundreds of gamma-detected blazars that have been observed spectroscopically in the optical. This allows to compare the jet power and the accretion luminosity in a robust way, to show that the two quantities correlate, and yet the jet power is dominant. This supports the view that the jet power originates from the extraction of the rotational energy of a fastly spinning hole, made possible by the torque exerted by a relatively strong magnetic field, in turn sustained and amplified by the accreting matter, in full agreement with recent numerical simulations.