Particle diffusion and localized acceleration in inhomogeneous AGN jets - Part I: Steady-state spectra

TL;DR

This paper models particle acceleration and emission in inhomogeneous AGN jets, showing how localized acceleration regions and diffusion influence the spectral features, potentially explaining observed hard gamma-ray spectra.

Contribution

It introduces a 2D inhomogeneous leptonic model for steady-state spectra in AGN jets, emphasizing the role of localized acceleration and diffusion in spectral formation.

Findings

Small acceleration regions can produce high-energy particles.

Diffusive escape explains the spectral shape of jet emission.

Inhomogeneity leads to harder SSC spectra than synchrotron spectra.

Abstract

We study the acceleration, transport, and emission of particles in relativistic jets. Localized stochastic particle acceleration, spatial diffusion, and synchrotron as well as synchrotron self-Compton emission are considered in a leptonic model. To account for inhomogeneity, we use a 2D axi-symmetric cylindrical geometry for both relativistic electrons and magnetic field. In this first phase of our work, we focus on steady-state spectra that develop from a time-dependent model. We demonstrate that small isolated acceleration region in a much larger emission volume are sufficient to accelerate particles to high energy. Diffusive escape from these small regions provides a natural explanation for the spectral form of the jet emission. The location of the acceleration regions within the jet is found to affect the cooling break of the spectrum in this diffusive model. Diffusion-caused energy-dependent inhomogeneity in the jets predicts that the SSC spectrum is harder than the synchrotron spectrum. There can also be a spectral hardening towards the high-energy section of the synchrotron spectrum, if particle escape is relatively slow. These two spectral hardening effects indicate that the jet inhomogeneity might be a natural explanation for the unexpected hard {\gamma}-ray spectra observed in some blazars.