Variability in Active Galactic Nuclei from Propagating Turbulent Relativistic Jets

TL;DR

This study models relativistic jets in active galactic nuclei using hydrodynamics simulations to analyze variability in synchrotron emission, revealing different power spectral density slopes for turbulence and bulk flow effects.

Contribution

It introduces a combined simulation and analysis approach to distinguish turbulence-induced and bulk flow-induced variability in AGN jets.

Findings

Power spectral densities range from -1.8 to -2.3 for turbulence.

Power spectral densities range from -2.1 to -2.9 for bulk velocity variations.

Turbulence dominates short-term variability, bulk flow dominates long-term variability.

Abstract

We use the Athena hydrodynamics code to model propagating two-dimensional relativistic jets as approximations to the growth of radio-loud active galactic nuclei for various input jet velocities and jet-to-ambient matter density ratios. Using results from these simulations we estimate the changing synchrotron emission by summing the fluxes from a vertical strip of zones behind the reconfinement shock, which is nearly stationary, and from which a substantial portion of the flux variability should arise. We explore a wide range of time scales by considering two light curves from each simulation; one uses a relativistic turbulence code with bulk velocities taken from our simulations as input, while the other uses the bulk velocity data to compute fluctuations caused by variations in the Doppler boosting due to changes in the direction and the speed of the flow through all zones in the strip. We then calculate power spectral densities (PSDs) from the light curves for both turbulent and bulk velocity origins for variability. The range of the power-law slopes of the PSDs for the turbulence induced variations is -1.8 to -2.3, while for the bulk velocity produced variations this range is -2.1 to -2.9; these are in agreement with most observations. When superimposed, these power spectra span a very large range in frequency (about five decades), with the turbulent fluctuations yielding most of the shorter timescale variations and the bulk flow changes dominating the longer periods.