Powerful high-energy emission of the remarkable BL Lac object S5 0716+714

Abstract

BL Lac objects of the intermediate subclass (IBLs) are known to emit a substantial fraction of their power in the energy range 0.1-10 GeV. Detecting γ-ray emission from such sources provides therefore a direct probe of the emission mechanisms and of the underlying powerhouse. The γ-ray satellite, AGILE, detected the remarkable IBL S5 0716+714 (z ≃ 0.3) during a high state in the period from 2007 September-October, marked by two very intense flares reaching peak fluxes of 200 × 10-8 photons cm -2 s-1 above 100 MeV, with simultaneous optical and X-ray observations. We present here a theoretical model for the two major flares and discuss the overall energetics of the source. We conclude that 0716+714 is among the brightest BL Lac's ever detected at γ-ray energies. Because of its high power and lack of signs for ongoing accretion or surrounding gas, the source is an ideal candidate to test the maximal power extractable from a rotating supermassive black hole via the pure Blandford-Znajek (BZ) mechanism. We find that during the 2007 γ-ray flares 0716+714 approached or just exceeded the upper limit set by BZ for a black hole of mass 109 M·. © 2009. The American Astronomical Society.