  Theoretical models for the production of relativistic jets from active
galactic nuclei predict that jet power arises from the spin and mass of the
central black hole, as well as the magnetic field near the event horizon. The
physical mechanism mechanism underlying the contribution from the magnetic
field is the torque exerted on the rotating black hole by the field amplified
by the accreting material. If the squared magnetic field is proportional to the
accretion rate, then there will be a correlation between jet power and
accretion luminosity. There is evidence for such a correlation, but inadequate
knowledge of the accretion luminosity of the limited and inhomogeneous used
samples prevented a firm conclusion. Here we report an analysis of archival
observations of a sample of blazars (quasars whose jets point towards Earth)
that overcomes previous limitations. We find a clear correlation between jet
power as measured through the gamma-ray luminosity, and accretion luminosity as
measured by the broad emission lines, with the jet power dominating over the
disk luminosity, in agreement with numerical simulations. This implies that the
magnetic field threading the black hole horizon reaches the maximum value
sustainable by the accreting matter.
