# Nonthermal emission in the lobes of Fornax A

## TL;DR

This paper re-evaluates the origin of gamma-ray emission in Fornax A's lobes, showing that energetic electrons alone can explain the observations without requiring a high proton contribution, contrary to previous claims.

## Contribution

The study provides a detailed calculation demonstrating that electron scattering accounts for the gamma-ray emission, challenging prior assumptions about proton dominance in the lobes.

## Key findings

- Electron scattering explains all observed gamma-ray emission.
- Optical radiation energy density is higher than previously estimated.
- Proton contribution to gamma-ray emission is not necessary.

## Abstract

Current measurements of the spectral energy distribution in radio, X-and-gamma-ray provide a sufficiently wide basis for determining basic properties of energetic electrons and protons in the extended lobes of the radio galaxy Fornax A. Of particular interest is establishing observationally, for the first time, the level of contribution of energetic protons to the extended emission observed by the Fermi satellite. Two recent studies concluded that the observed gamma-ray emission is unlikely to result from Compton scattering of energetic electrons off the optical radiation field in the lobes, and therefore that the emission originates from decays of neutral pions produced in interactions of energetic protons with protons in the lobe plasma, implying an uncomfortably high proton energy density. However, our exact calculation of the emission by energetic electrons in the magnetized lobe plasma leads to the conclusion that all the observed emission can, in fact, be accounted for by energetic electrons scattering off the ambient optical radiation field, whose energy density (which, based on recent observations, is dominated by emission from the central galaxy NGC 1316) we calculate to be higher than previously estimated.