A numerical study of the interplay between Fermi acceleration mechanisms in radio lobes of FR-II radio galaxies

Abstract

Context. Radio-loud active galactic nuclei (AGNs) are thought to possess various sites of particle acceleration, which gives rise to the observed nonthermal spectra. Stochastic turbulent acceleration (STA) and diffusive shock acceleration (DSA) are commonly cited as potential sources of high-energy particles in weakly magnetized environments. Together, these acceleration processes and various radiative losses determine the emission characteristics of these extragalactic radio sources. Aims. The purpose of this research is to investigate the dynamical interplay between the STA and DSA in the radio lobes of FR-II radio galaxies, as well as the manner in which these acceleration mechanisms, along with a variety of radiative losses, collectively shape the emission features seen in these extragalactic sources. Methods. A phenomenologically motivated model of STA is considered and subsequently employed on a magnetohydrodynamically simulated radio lobe through a novel hybrid Eulerian-Lagrangian framework. Results. STA gives rise to a curved particle spectrum that is morphologically different from the usual shock-accelerated spectrum. As a consequence of this structural difference in the underlying particle energy spectrum, various multiwavelength features arise in the spectral energy distribution of the radio lobe. Additionally, we observe enhanced diffuse X-ray emission from radio lobes for cases where STA is taken into account in addition to DSA.