On the role of black hole spin and accretion in powering relativistic jets in AGN

Motivated by the remarkable results on the general properties of blazars obtained from Fermi observations we discuss the role of spin and accretion in powering relativistic jets in AGNs. We consider studies of the Blandford-Znajek mechanism taking into account advection of magnetic flux from an accretion disk surrounding the Black Hole (BH) which indicate that the mechanism is inhibited for values of the BH spin a ∼< 0.5. However large scale propagation of the jet with highly relativistic bulk acceleration likely requires even larger spin values. The Blandford-Znajek mechanism then predicts a tight connection between jet power and accretion power, accounting for the correlation between gamma-ray luminosity and accretion disk luminosity found in the bright Fermi blazar sample. The suggested spin threshold would lead to a grand unification of AGN based on only two intrinsic parameters: the BH spin and the accretion rate in Eddington units.