Flare echos from relaxation shocks in perturbed relativistic jets
  One of the main scenarios to account for the multi-wavelength flux
variability observed in relativistic jets of AGNs is based on diffusive shock
acceleration of a population of relativistic electrons on internal shocks of
various origins. Any complete AGN emission scenario has to be able to explain
the wide range of observed variability time scales between the radio and
gamma-ray band. In addition constraints are also provided by very-long-baseline
interferometry (VLBI), which shows a large variety of moving and standing
emission zones with distinct behaviors. We aim to characterize the evolution of
stationary and moving emission zones in the jet and to study their
multi-wavelength signatures through emission maps and light curves. We focus
our study on flare events that occur during strong interactions between moving
ejecta and stationary recollimation shocks. We simulate relativistic jets with
the magneto-hydrodynamic code MPI-AMRVAC and inject non-thermal particle
distributions of electrons in shock regions. We follow the propagation of a
moving shock and its interactions with a structure of standing re-collimation
shocks in the jet. Synchrotron emission and radiative transfer are calculated
in the post-processing code RIPTIDE for given observation angles and
frequencies. We demonstrate the appearance of trailing components behind the
leading moving shock. The latter destabilizes the jet, causing the emergence of
oscillating standing shocks and relaxation shocks. Emissions from these regions
can dominate the overall flux or lead to flare echos in the light curve.
Another observational marker of relaxation shocks appears in time-distance
plots of bright VLBI components of the jet. Our scenario provides a plausible
explanation for radio VLBI observations of the radio-galaxy 3C 111 where
trailing components have been observed during a radio outburst event in 1997.
