Variation of the γγ opacity by the Heii Lyman continuum constrains the location of the γ-ray emission region in the blazar 3C454.3

Abstract

We study spectral properties of the brightest γ-ray blazar 3C454.3 using 138 weeks of observations by the Fermi Gamma-ray Space Telescope (Fermi). We probe the behaviour of the source as a function of time at different brightness levels. The Fermi spectra in the GeV range can be well described by a wide underlying lognormal distribution with the photon-photon absorption breaks produced by the Heii and Hi Lyman recombination continua (LyC). We find a power-law dependence of the peak energy on flux and discover anticorrelation between the column density of the Heii LyC and flux. This implies that the γ-ray emission zone lies close to the boundary of the high-ionization part of the broad-line region and moves away from the black hole when the flux increases. Identification of the γ-ray production with the relativistic jet implies that the jet is already accelerated at sub-parsec distances from the central black hole, which favours the Blandford-Znajek process as the jet launching mechanism. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS.