Simulating the dynamics and synchrotron emission from relativistic jets II. Evolution of non-thermal electrons

TL;DR

This study simulates the evolution of non-thermal cosmic ray electrons in 3D relativistic jets, revealing how jet stability affects electron acceleration, spectral complexity, and energy distribution within the jet and cocoon regions.

Contribution

It provides a detailed simulation of CRE evolution in relativistic jets, including effects of instabilities, shock re-acceleration, and spectral mixing, advancing understanding of non-thermal processes in astrophysical jets.

Findings

Unstable jets create complex shock structures and re-accelerate CREs.

CRE spectra are highly variable and depend on jet stability and turbulence.

Cocoons of unstable jets accumulate energetic electrons, affecting non-thermal energy budgets.

Abstract

We have simulated the evolution of non-thermal cosmic ray electrons (CREs) in 3D relativistic magneto hydrodynamic (MHD) jets evolved up to a height of 9 kpc. The CREs have been evolved in space and in energy concurrently with the relativistic jet fluid, duly accounting for radiative losses and acceleration at shocks. We show that jets stable to MHD instabilities show expected trends of regular flow of CREs in the jet spine and acceleration at a hotspot followed by a settling backflow. However, unstable jets create complex shock structures at the jet-head (kink instability), the jet spine-cocoon interface and the cocoon itself (Kelvin-Helmholtz modes). CREs after exiting jet-head undergo further shock crossings in such scenarios and are re-accelerated in the cocoon. CREs with different trajectories in turbulent cocoons have different evolutionary history with different spectral parameters. Thus at the same spatial location, there is mixing of different CRE populations, resulting in a complex total CRE spectrum when averaged over a given area. Cocoons of unstable jets can have an excess build up of energetic electrons due to re-acceleration at turbulence driven shocks and slowed expansion of the decelerated jet. This will add to the non-thermal energy budget of the cocoon.