The power form BL Lacs

Blazars are among the brightest Active Galactic Nuclei (AGNs) observed with inferred (isotropic) luminosities up to L_iso ~ 10^47 erg s-1. Emission from these sources is widely held to originate from a relativistic jet closely aligned to the observer line of sight, with bulk Lorentz factor Γ ~ 10 ÷ 20. Taking into account related relativistic effects the intrinsic jet powers are actually much lower, L ~ Γ^-2 L_iso. A physical reference is provided by the power extractable from a central rotating Kerr Hole spun up by past accretion via the electromagnetic interaction with the magnetic field of the accretion disc, the Blandford-Znajek (BZ) mechanism. The limiting power reads L_BZ ~ 2 x 10^45 (M_BH/10^9 M-sun) erg s-1 in an equilibrium magnetic field B ~ 10^4 G. We study in particular gas-poor BL Lacs, which show no evidence of current accretion or well developed accretion disk, and can therefore provide a simple benchmark for this model. A study of several such BL Lacs across the entire electromagnetic spectrum allows robust estimates of the source parameters and energetics, showing that these sources comply with the BZ limit or just exceed it, within the uncertainties on black hole (BH) masses estimation. In particular, powerful BL Lacs like 0716+714 close to the BZ limit appear to be constrained in the evolution of their flaring activity. It is therefore exciting to look for sources sharply exceeding the BZ limit. If any, they could involve orbits plunging into the BH horizon in a region with very intense magnetic field B > 10^4 G and dominated by strong gravity effects.