  High-energy photons propagating in the magnetised medium with large velocity
gradients can mediate energy and momentum exchange. Conversion of these photons
into electron-positron pairs in the field of soft photons with the consequent
isotropization and emission of new high-energy photons by Compton scattering
can lead to the runaway cascade of the high-energy photons and
electron-positron pairs fed by the bulk energy of the flow. This is the essence
of the photon breeding mechanism. We study the problem of high-energy emission
of relativistic jets in blazars via photon breeding mechanism using 2D
ballistic model for the jet with the detailed treatment of particle propagation
and interactions. The gamma-ray background of similar energy density as
observed at Earth is sufficient to trigger the photon breeding. As a result, a
jet can convert up to 80 per cent of its total power into radiation. Photon
breeding produces a population of high-energy pairs and predicts the spectra in
agreement with observations of blazars (e.g. the blazar sequence). It also
decelerates the jet at subparsec scales and induces the transversal gradient of
the Lorentz factor which reconcile the discrepancy between the high Doppler
factors determined from the spectra of TeV blazars and the low apparent
velocities observed at VLBI scales. The broad angular distribution of radiation
predicted by the mechanism reconciles the observed statistics and luminosity
ratio of FR I and BL Lac objects with the large Lorentz factors of the jets as
well as explains the high level of the TeV emission in the radio galaxy M87.
(abridged)
