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

TL;DR

This study addresses how flickering AGN jet power shapes kpc-scale radio emission by running 3D relativistic hydrodynamics simulations with the PLUTO code and a Lagrangian tracer-based electron population evolution. A novel adiabatic cooling treatment via a fluid tracer mitigates mixing artifacts, and jet power variability follows a pink-noise, lognormal process with S(f) ∝ f^{-1} and σ = 0.33, enabling realistic light curves for Q and associated Lorentz factors Γ_j. Key results show that rapid power increases drive travelling shocks that interact with the jet-head's double-shock structure, producing dramatic hotspot brightening (up to ~10×) and occasional traveling emission patches along the jet, while time-averaged radiative efficiency remains similar to steady jets; thus variability can explain part of the diversity seen in radio jet morphologies and luminosities. The findings imply that instantaneous hotspot luminosities may reflect recent accretion history rather than just current jet power, affecting energy budgets and P–D diagrams, and highlight observational signatures to search for flickering in radio surveys. The work lays groundwork for incorporating variability into interpretations of jet feedback and for refining probes of accretion history using spatially resolved radio observations.

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.

Figures (17)

• Figure 1: Emissivity map, pressure and density of jet material for a portion of the $\sigma=0.33$, seed 12 simulation at 9 Myr. The pressure and density maps show slices through the centre of the jet, whilst the emissivity map is summed along a line of sight perpendicular to the jet axis. The full simulation domain extends to 60 kpc in the jet direction and between -30 and 30 kpc in the two directions perpendicular to this. The locations and velocities of a small random sample of Lagrangian particles from the central slice through the domain are shown overlaid on the density of jet material. Velocities are shown as arrows where the length of the arrow scales linearly with speed.
• Figure 2: Input jet powers and corresponding Lorentz factors over time, with median jet powers and Lorentz factors over the 10 Myr sample shown by dashed lines (see also Table \ref{['tab:sim_vars']}). There is an approximately linear scaling between $\Gamma$ and $Q$ when $\Gamma\approx2$.
• Figure 3: Passive scalar (fluid) tracer of jet material from central slice through the $10^{45}$ erg s$^{-1}$ constant power simulation at 7 Myr. Mixing of the low density material which enters the simulation through the jet with high density material originating in the ambient medium leads to very low fractions of jet material in the lobe regions.
• Figure 4: Emissivity maps for the 8-10 Myr period of the constant power $10^{45}$ erg s$^{-1}$ jet and the three variable power jets with seeds 0, 6 and 12. The constant power jet has a similar morphology over this full time period with two to three clear recollimation shocks which transition into a turbulent region followed by a modest hotspot. In contrast, the variable jets show a large amount of variety in their morphologies over time. The recollimation shocks move to larger and smaller distances from the jet launch point and the hotspot luminosity changes dramatically over the 2 Myr, sometimes being much brighter than the hotspots in the constant power case. See supplementary material for videos of the variable jets (https://youtu.be/cen9ZF1PPRM, https://youtu.be/mk9wSW6_E_Q, https://youtu.be/113B7f87fRA).
• Figure 5: Luminosity at 144 MHz as a function of distance along the jet for the jet with constant power $10^{45}$ erg s$^{-1}$. Lines are offset from zero depending on jet age with earlier times in the simulation shown further up the axes. The emissivity maps are convolved with a Gaussian filter with a standard deviation of 5 prior to integration.