Application of the spine-layer jet radiation model to outbursts in the broad-line radio galaxy 3C 120

TL;DR

This paper analyzes gamma-ray flares in the radio galaxy 3C 120 using Fermi/LAT data, modeling jet structure to explain rapid variability and spectral energy distribution during outbursts.

Contribution

It introduces a spine-layer jet radiation model to explain fast gamma-ray variability and spectral features in 3C 120 during flares.

Findings

Fast variability timescale of hours during flares

Gamma-ray emission explained by external Compton and synchrotron self-Compton processes

Jet model with a fast spine and slow outer layer accounts for observed spectra

Abstract

We present a detailed Fermi/LAT data analysis for the broad-line radio galaxy 3C 120. This source has recently entered into a state of increased gamma-ray activity which manifested itself in two major flares detected by Fermi/LAT in September 2014 and April 2015 with no significant flux changes reported in other wavelengths. We analyse available data focusing our attention on aforementioned outbursts. We find very fast variability timescale during flares (of the order of hours) together with a significant gamma-ray flux increase. We show that the 6.8 years averaged gamma-ray emission of 3C 120 is likely a sum of the external radiation Compton and the synchrotron self-Compton radiative components. To address the problem of "orphan" gamma-ray flares and fast variability we model the jet radiation dividing the jet structure into two components: the wide and relatively slow outer layer and the fast, narrow spine. We show that with the addition of the fast spine occasionally bent towards the observer we are able to explain observed spectral energy distribution of 3C 120 during flares with the Compton up-scattered broad-line region and dusty torus photons as main gamma-rays emission mechanism.