The mystery of spectral breaks: Lyman continuum absorption by
  photon-photon pair production in the Fermi GeV spectra of bright blazars
  We reanalyze Fermi/LAT gamma-ray spectra of bright blazars with a higher
photon statistics than in previous works and with new Pass 7 data
representation. In the spectra of the brightest blazar 3C 454.3 and possibly of
4C +21.35 we detect breaks at 5 GeV (in the rest frame) associated with the
photon-photon pair production absorption by He II Lyman continuum (LyC). We
also detect confident breaks at 20 GeV associated with hydrogen LyC both in the
individual spectra and in the stacked redshift-corrected spectrum of several
bright blazars. The detected breaks in the stacked spectra univocally prove
that they are associated with atomic ultraviolet emission features of the
quasar broad-line region (BLR). The dominance of the absorption by hydrogen Ly
complex over He II, rather small detected optical depth, and the break energy
consistent with the head-on collisions with LyC photons imply that the
gamma-ray emission site is located within the BLR, but most of the BLR emission
comes from a flat disk-like structure producing little opacity. Alternatively,
the LyC emission region size might be larger than the BLR size measured from
reverberation mapping, and/or the gamma-ray emitting region is extended. These
solutions would resolve a long-standing issue how the multi-hundred GeV photons
can escape from the emission zone without being absorbed by softer photons.
