Dynamic changes of emitting electron distribution in the jet of 3C 279: signatures of acceleration and cooling

TL;DR

This study analyzes the electron energy distribution changes in 3C 279's jet across different states, revealing correlations that support a stochastic acceleration scenario with a balance between acceleration and cooling.

Contribution

It introduces a systematic MCMC modeling of 14 SEDs of 3C 279 using a three-parameter log-parabola EED, providing insights into electron acceleration and cooling processes.

Findings

Positive correlation between EED curvature and electron peak energy

External Compton scattering of dust radiation explains gamma-ray spectra

No clear correlation between EED curvature and synchrotron peak frequency

Abstract

We study the dynamic changes of electron energy distribution (EED) through systematically analysing the quasi-simultaneous spectral energy distributions (SEDs) of the flat spectrum radio quasar 3C 279 in different states. With Markov chain Monte Carlo (MCMC) technique we model fourteen SEDs of 3C 279 using a leptonic model with a three-parameter log-parabola electron energy distribution (EED). The 14 SEDs can be satisfactorily fitted with the one-zone leptonic model. The observed $\gamma$ rays in 13 states are attributed to Compton scattering of external infrared photons from a surrounding dusty torus. The curved $\gamma$-ray spectrum observed during 2-8 April 2014 is well explained by the external Compton of dust radiation. It is found that there is a clear positive correlation between the curvature parameter $b$ of the EED and the electron peak energy $\gamma'_{\rm pk}$. No clear correlation between $b$ and the synchrotron peak frequency $\nu_{\rm s}$ is found, due to the varied product of Doppler factor and fluid magnetic field from state to state. We interpret the correlation of $b-\gamma'_{\rm pk}$ in a stochastic acceleration scenario. This positive correlation is in agreement with the prediction in the stage when the balance between acceleration and radiative cooling of the electrons is nearly established in the case of the turbulence spectral index $q=2$.