Letter to the Editor

Power for dry BL Lacertae objects

¹ Dipartimento di Fisica, Università di Roma “Tor Vergata”, and INFN Roma Tor Vergata via della Ricerca Scientifica 1, 00133 Roma, Italy e-mail: paggi@roma2.infn.it
² INAF/IASF-Roma, via Fosso del Cavaliere 1, 00100 Roma, Italy

Received: 28 October 2009
Accepted: 16 November 2009

Abstract

Is it significant that the intrinsic outputs of several BL Lacs are observed to level off at values of about  erg s^-1? 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  photons cm^-2 s^-1 in the  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  erg s^-1, 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 , implying that the source is significantly gauged in terms of the maximal power around  erg s^-1 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.