Simulating radio emission from flickering AGN jets: travelling shocks and hotspot brightening

TL;DR

This study uses relativistic hydrodynamics simulations to explore how flickering variability in AGN jet power affects radio galaxy luminosity, morphology, and hotspot brightness, revealing that rapid jet power changes can significantly influence observable features.

Contribution

The paper introduces an improved Lagrangian particle method for tracking electron spectra in jet simulations, accounting for adiabatic cooling and jet variability effects.

Findings

Rapid jet power increases cause hotspot luminosity boosts.

Variable jets have similar average radiative efficiency to steady jets.

Instantaneous radiative efficiency shows significant variability.

Abstract

We investigate the impact of flickering variability in jet power on the luminosity and morphology of radio galaxies. We use a Lagrangian particle method together with relativistic hydrodynamics simulations using the PLUTO code to track the evolution of electron spectra through particle acceleration at shocks and cooling processes. We introduce an adapted version of this method which improves tracking of adiabatic cooling in regimes where low density jet material mixes with high density from the ambient medium in the lobes. We find that rapid increases in jet power can lead to large increases in hotspot luminosity due to the interaction of a travelling shock structure with the pre-existing shock structure at the jet head. We show that in some cases it may be possible to identify a bright region of emission corresponding to a shock travelling along the jet axis. We find that the time-averaged radiative efficiency of variable jets is similar to their steady counterparts, but find significant departures from this on an instantaneous basis. We suggest that, together with environmental effects and differences in the average powers of jets, variable jet powers may have a significant impact on how we understand the diversity of radio jets seen in observations and have significant implications for interpretations of jet powers, energy budgets and luminosity-linear size diagrams.