A time-dependent particle acceleration and emission model: Understanding the particle spectral evolution and blazar flares

TL;DR

This paper presents a time-dependent particle evolution model for blazar emissions, incorporating variable electric and magnetic fields to explain spectral and light curve variability observed in blazar jets.

Contribution

The model introduces a novel approach by including time-dependent electromagnetic fields in the transport equation for electron evolution, explaining blazar variability.

Findings

Light curve profiles align with spectral evolution from time-dependent fields.

Model successfully reproduces variability of Mrk 421's spectrum and light curves.

Magnetic field evolution can account for observed blazar variability.

Abstract

The jets of blazars are renowned for their multi-wavelength flares and rapid extreme variability; however, there are still some important unanswered questions about the physical processes responsible for these spectral and temporal changes in emission properties. In this paper, we develop a time-dependent particle evolution model for the time-varying emission spectrum of blazars. In the model, we introduce time-dependent electric and magnetic fields, which consistently include the variability of relevant physical quantities in the transport equation. The evolution on the electron distribution is numerically solved from a generalized transport equation that contains the terms describing the electrostatic, first-order and second-order \emph{Fermi} acceleration, escape of particles due to both advection and spatial diffusion, as well as energy losses due to the synchrotron emission and inverse-Compton scattering of both synchrotron and external ambient photon fields. We find that the light curve profiles of blazars are consistent with the particle spectral evolution resulting from time-dependent electric and magnetic fields, rather than the effects of the acceleration or the cooling processes. The proposed model is able to simultaneously account for the variability of both the energy spectrum and the light curve profile of the BL Lac object Mrk 421 with reasonable assumptions about the physical parameters. The results strongly indicate that the magnetic field evolution in the dissipated region of a blazar jet can account for the variabilities.