Variation of the gamma-gamma opacity by the He II Lyman continuum
  constrains the location of the gamma-ray emission region in the blazar 3C
  454.3
  We study spectral properties of the brightest gamma-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 He II and H I Lyman recombination continua (LyC). We find a
power-law dependence of the peak energy on flux and discover anti-correlation
between the column density of the He II LyC and flux. This implies that the
gamma-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 gamma-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.
