Flare echos from relaxation shocks in perturbed relativistic jets

TL;DR

This paper models how interactions between moving shocks and stationary recollimation shocks in relativistic jets produce observable flare echos and trailing components, explaining multi-wavelength variability and VLBI observations.

Contribution

It introduces a simulation framework combining MHD and radiative transfer to analyze shock interactions and their emission signatures in AGN jets.

Findings

Trailing components behind shocks can cause flare echos.

Interactions destabilize jets, creating oscillating shocks.

Simulation matches VLBI observations of 3C 111.

Abstract

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.