Spectral evolution of flaring blazars from numerical simulations
  High resolution Very Long Baseline Interferometry (VLBI) observations of
Active Galactic Nuclei (AGN) revealed traveling and stationary or
quasi-stationary radio-components in several blazar jets. The traveling ones
are in general interpreted as shock waves generated by pressure perturbations
injected at the jet nozzle. The stationary features can be interpreted as
recollimation shocks in non-pressure matched jets if they show a
quasi-symmetric bump in the spectral index distribution. In some jets there may
be interactions between the two kinds of shocks. These shock--shock
interactions are observable with VLBI techniques, and their signature should
also be imprinted on the single--dish light curves. We performed relativistic
hydrodynamic (RHD) simulations of over-pressured and pressure-matched jets. To
simulate the shock interaction we injected a perturbation at the jet nozzle
once a steady-state was reached. We computed the non-thermal emission
(including adiabatic and synchotron losses) resulting from the simulation. We
show that the injection of perturbations in a jet can produce a bump in
emission at GHz frequencies previous to the main flare, which is produced when
the perturbation fills the jet in the observer's frame. The detailed analysis
of our simulations and the non-thermal emission calculations show that
interaction between a recollimation shock and traveling shock produce a typical
and clear signature in both the single--dish light curves and in the VLBI
observations: the flaring peaks are higher and delayed with respect to the
evolution of a perturbation through a conical jet. This fact can allow to
detect such interactions for stationary components lying outside of the region
in where the losses are dominated by inverse Compton scattering.
