Power for dry BL Lacertae objects

Abstract

Is it significant that the intrinsic outputs of several BL Lacs are observed to level off at values of about 10^46 erg s^-1? In searching for an answer, we compare gamma-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 2 x 10^-6 photons cm^-2 s^-1 in the 0.1-10 GeV 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 3 x 10^45 erg s^-1, which makes it one of the brightest dry BL Lacs so far detected in gamma rays. We evaluate the mass of the associated Kerr hole to be around 5 x 10^8 M_sun, implying that the source is significantly gauged in terms of the maximal power around 4 x 10^45 erg s^-1 extractable via the Blandford-Znajek electrodynamical mechanism; other dry BL Lacs observed in gamma 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.

4 pages, 3 figures, accepted by A&A

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