BL Lacs bright in rays

In this preview we consider implications from the recent AGILE γ -ray detection of the BL Lac 0716+714 in September–October 2007, marked by two intense flaring episodes reaching peak fluxes of 200×10−8 photons cm−2 s−1 in the energy range 0.1–10 GeV. The source shows no evidence of emission lines or disk radiation, related to any surrounding gas; its pure non-thermal radiation is effectively represented in terms of the synchrotron-self Compton radiation in the Thomson regime, also supported by the observed quadratic relation of the γ -ray to the X-ray flux variations. With source parameters so derived, we find a total jet power of about 4×1045 erg s−1, that makes 0716+714 one of the brightest gas-poor BL Lacs so far detected at γ -ray energies. Thus it provides an ideal benchmark to compare its intrinsic luminosity with the top power extractable from a maximally rotating black-hole via the Blandford–Znajek mechanism. With a mass close to 5 × 10 8 M ⊙ for the associated BH, we find the source to remain close to the Blandford–Znajek threshold during the flare. Other gas-poor but weaker BL Lacs remain well below the threshold. These findings and those expected from FERMI will provide a powerful test of GR at work.