Flux variability from ejecta in structured relativistic jets with large-scale magnetic fields

Abstract

Context. Standing and moving shocks in relativistic astrophysical jets are very promising sites for particle acceleration to large Lorentz factors and for the emission from the radio up to the γ-ray band. They are thought to be responsible for at least part of the observed variability in radio-loud active galactic nuclei. Aims. We aim to simulate the interactions of moving shock waves with standing recollimation shocks in structured and magnetized relativistic jets and to characterize the profiles of connected flares in the radio light curve. Methods. Using the relativistic magneto-hydrodynamic code MPI-AMRVAC and a radiative transfer code in post-processing, we explore the influence of the magnetic-field configuration and transverse stratification of an over-pressured jet on its morphology, on the moving shock dynamics, and on the emitted radio light curve. First, we investigate different large-scale magnetic fields with their effects on the standing shocks and on the stratified jet morphology. Secondly, we study the interaction of a moving shock wave with the standing shocks. We calculated the synthetic synchrotron maps and radio light curves and analyze the variability at two frequencies 1 and 15.3 GHz and for several observation angles. Finally, we compare the characteristics of our simulated light curves with radio flares observed from the blazar 3C 273 with the Owens Valley Radio Observatory and Very Long Baseline Array in the MOJAVE survey between 2008 and 2019. Results. We find that in a structured over-pressured relativistic jet, the presence of the large-scale magnetic field structure changes the properties of the standing shock waves and leads to an opening in the jet. The interaction between waves from inner and outer jet components can produce strong standing shocks. When crossing such standing shocks, moving shock waves accompanying overdensities injected in the base of the jet cause very luminous radio flares. The observation of the temporal structure of these flares under different viewing angles probes the jet at different optical depths. At 1 GHz and for small angles, the self-absorption caused by the moving shock wave becomes more important and leads to a drop in the observed flux after it interacts with the brightest standing knot. A weak asymmetry is seen in the shape of the simulated flares, resulting from the remnant emission of the shocked standing shocks. The characteristics of the simulated flares and the correlation of peaks in the light curve with the crossing of moving and standing shocks favor this scenario as an explanation of the observed radio flares of 3C 273.

Flare echos from relaxation shocks in perturbed relativistic jets

“…To trigger internal re-collimation shock patterns inside the jet, we assume that the jet is over-pressured compared to the ambient medium with a fixed pressure ratio of p jet /p am = 1.5 (Gómez et al 1997) and a ratio of the rest mass density of ρ jet /ρ am = 10 −2 . As we apply a constant density and pressure profile in the ambient medium and as the jet inlet is cylindrical, we expect to observe only an intrinsic jet opening angle of small amplitude (Fichet de Clairfontaine et al 2021). While this configuration does not reproduce the jet opening that is observed on the average (Pushkarev et al 2017), the objective here is to study the strong interaction of the moving shock with multiple stationary shocks.…”

“…In the γ-ray range, these knots cannot be resolved, but the high-energy emission is usually attributed, at least in part, to compact regions inside the jet, due to the observed very rapid variability. Quasi-stationary knots may be interpreted as re-collimation shocks, naturally occurring in overpressured jets (Marscher et al 2008;Fromm et al 2013a;Hervet et al 2017;Fichet de Clairfontaine et al 2021). This mechanism is for example widely accepted to explain the HST-1 radio knot in the nearby radio-galaxy M 87 (Stawarz et al 2006).…”

“…So far, most hydrodynamic studies of shock-shock interactions in jets have focused on interactions of weak, moving perturbations with standing or moving shocks (Mimica et al 2009;Fromm et al 2016;Vaidya et al 2018;Winner et al 2019;Fichet de Clairfontaine et al 2021;Huber, D. et al 2021). In this paper, the study of the interaction between strong, moving shocks with successive re-collimation shocks allows us to investigate the perturbation of the standing shock structure and the emergence of relaxation shocks, which can arise after strong shock-shock interactions.…”

Electron-proton co-acceleration on relativistic shocks in extreme-TeV blazars

“…The shock is thus mildly relativistic for κ of the order of unity. The angle θ j< − α may however show a different behaviour in jets where recollimation shocks arise from a difference in pressure between jet components and the ambient medium (e.g., Fichet de Clairfontaine et al 2021).…”

ExHaLe-jet: An extended hadro-leptonic jet model for blazars. I. Code description and initial results

“…This demands radiation models beyond the one-zone model. While MHD, RMHD and GRMHD codes have improved (and continue to do so) to model jets on vast scales (e.g., Chatterjee et al 2020;Dong et al 2020;Fichet de Clairfontaine et al 2021), the efficient calculation of all kinds of radiation processes (for a recent review, see Cerruti 2020) is best done with kinetic models. In such models, the kinetic equation governing the particle distribution under influences of acceleration, cooling and other losses, is solved along the jet flow by cutting the jet into numerous slices and imposing a fixed jet geometry and bulk-flow evolution.…”