2009A&A...508L..31P

Is it significant that the intrinsic outputs of several BL Lacs are observed to level off at values of about 10⁴⁶erg/s In searching for an answer, we compare γ-ray observations by the AGILE satellite of the BL Lac S5 0716+714 with those of Mrk 421 and Mrk 501; the former are particularly marked by intense flares up to fluxes of 2x10^–6photons/cm2/s in the 0.1-10GeV energy range. These ``dry'' BL Lacs show evidence of neither thermal disk emissions nor emission lines signaling any accreting or surrounding gas; the spectral distributions of their pure non-thermal radiations are effectively represented by the synchrotron self-Compton process. With source parameters correspondingly derived and tuned with simultaneous multiwavelength observations, we find for S5 0716+714 a total jet power of about 3x10⁴⁵erg/s, which makes it one of the brightest dry BL Lacs so far detected in γ rays. We evaluate the mass of the associated Kerr hole to be around 5x10⁸M_☉, implying that the source is significantly gauged in terms of the maximal power around 4x10⁴⁵erg/s extractable via the Blandford-Znajek electrodynamical mechanism; other dry BL Lacs observed in γ rays remain well below that threshold. These findings and those forthcoming from Fermi-LAT will provide a powerful test of electrodynamics in the surroundings of the hole, that are dominated by GR effects.