Gamma-rays in the radio galaxy 3C 84: A complex situation

TL;DR

This paper investigates the gamma-ray emission regions in the nearby radio galaxy 3C 84, revealing complex interactions near the jet launching site through multi-instrument VLBI observations and kinematic analysis.

Contribution

It combines gamma-ray data with high-resolution VLBI observations to localize emission regions and introduces a kinematic model explaining high-energy flares via shock interactions.

Findings

Gamma-ray emission occurs both near the jet base and downstream.

A fast-traveling shock interacting with a slower shock causes gamma-ray flares.

The jet launching region shows a broad double nuclear structure.

Abstract

3C 84 is a nearby Active Galactic Nucleus (AGN) that is unique in that is believed that we are observing near the true jet launching region - unlike blazars. The source is active in Gamma rays and has been detected with Fermi since its launch in 2008, including being detected at TeV energies with other instruments. Due to the relative proximity of the source (z=0.018), it provides a unique opportunity to pinpoint the location of the $\gamma$-ray emission by combining the Gamma ray data with very long baseline inteferometry (VLBI) data. A study using the Korean VLBI network (KVN) showed that the Gamma rays occur in both downstream jet emission and the region near where the jet is launched. Further analysis of the kinematics using Wavelet Image Segmentation and Evaluation (WISE) algorithm, which uses 2-dimensional cross-correlations to statistically derive the kinematics of high-resolution 7 mm VLBA data show that the Gamma ray emission is caused by a fast-travelling shock catching a slower moving shock and then interacting with the external medium, in behaviour reminiscent of a long duration gamma-ray burst (GRB). This could explain why such high energy flaring is seen in such low Doppler boosted sources. Finally, we show some early results from a study of the jet launching region using the Global mm-VLBI Array (GMVA). The nucleus appears to have a consistent double nuclear structure that is likely too broad to be the true jet base.