Variations of the GeV opacity in the gamma-ray blazar 3C 454.3

Abstract

We study spectral properties of the brightest GeV blazar 3C 454.3 using the Fermi data for 137 weeks of observations. We probe the behaviour of the source as a function of time at different brightness levels. The spectra in the GeV range can be well described by a wide underlying lognormal distribution with photon-photon absorption breaks produced by He II and H I recombination continua. We confirm the flux--hardness correlation at a high significance level and discover the anti-correlation between the flux and the optical depth in He II photons at a marginally significant level. This implies that the gamma-ray emission region lies close to the boundary of the high-ionization zone of BLR and moves away from the black hole when the flux increases. Identification of the gamma-ray emission zone with the relativistic jet, implies that the jet is already accelerated at sub-parsec distances from the central black hole. This rules out the jet launching mechanism by the accretion disc, but instead favours the Blandford-Znajek process.