  Axion-Like Particles (ALPs), if exist in nature, are expected to mix with
photons in the presence of an external magnetic field. The energy range of
photons which undergo strong mixing with ALPs depends on the ALP mass, on its
coupling with photons as well as on the external magnetic field and particle
density configurations. Recent observations of blazars by the Fermi Gamma-Ray
Space Telescope in the 0.1-300 GeV energy range show a break in their spectra
in the 1-10 GeV range. We have modeled this spectral feature for the
flat-spectrum radio quasar 3C454.3 during its November 2010 outburst, assuming
that a significant fraction of the gamma rays convert to ALPs in the large
scale jet of this blazar. Using theoretically motivated models for the magnetic
field and particle density configurations in the kiloparsec scale jet, outside
the broad-line region, we find an ALP mass $m_a\sim (1-3)\cdot 10^{-7}$ eV and
coupling $g_{a\gamma} \sim (1-3)\cdot 10^{-10}$ GeV$^{-1}$ after performing an
illustrative statistical analysis of spectral data in four different epochs of
emission. The precise values of $m_a$ and $g_{a\gamma}$ depend weakly on the
assumed particle density configuration and are consistent with the current
experimental bounds on these quantities. We apply this method and ALP
parameters found from fitting 3C454.3 data to another flat-spectrum radio
quasar PKS1222+216 (4C+21.35) data up to 400 GeV, as a consistency check, and
found good fit. We find that the ALP-photon mixing effect on the GeV spectra
may not be washed out for any reasonable estimate of the magnetic field in the
intergalactic media.
