Particle diffusion and localized acceleration in inhomogeneous AGN jets - Part II: stochastic variation

TL;DR

This paper models stochastic particle acceleration in inhomogeneous AGN jets to explain observed flux variability, spectral features, and timing correlations across different energy bands, providing insights into blazar emission mechanisms.

Contribution

It introduces a 2-D spatially resolved leptonic jet model that links particle acceleration events to flux variability and timing properties in blazar emissions, advancing understanding of jet inhomogeneity effects.

Findings

Spectral breaks in PSDs correspond to characteristic acceleration timescales.

Flux-flux correlations can be linear or quadratic, affecting flare lags.

Cooling effects influence the timing and spectral features of emissions.

Abstract

We study the stochastic variation of blazar emission under a 2-D spatially resolved leptonic jet model we previously developed. Random events of particle acceleration and injection in small zones within the emission region are assumed to be responsible for flux variations. In addition to producing spectral energy distributions that describe the observed flux of Mrk 421, we further analyze the timing properties of the simulated light curves, such as the power spectral density (PSD) at different bands, flux-flux correlations, as well as the cross-correlation function between X-rays and TeV {\gamma}-rays. We find spectral breaks in the PSD at a timescale comparable to the dominant characteristic time scale in the system, which is usually the pre-defined decay time scale of an acceleration event. Cooling imposes a delay, and so PSDs taken at lower energy bands in each emission component (synchrotron or inverse Compton) generally break at longer timescales. The flux-flux correlation between X-rays and TeV {\gamma}-rays can be either quadratic or linear, depending on whether or not there are large variation of the injection into the particle acceleration process. When the relationship is quadratic, the TeV flares lag the X-ray flares, and the optical & GeV flares are large enough to be comparable to the ones in X-ray. When the relationship is linear, the lags are insignificant, and the optical & GeV flares are small.