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

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 $q=2$.