Non-thermal emission in radio galaxy lobes: II. Centaurus A, Centaurus B, and NGC 6251

TL;DR

This paper analyzes radio, X-ray, and gamma-ray data from the lobes of Centaurus A, B, and NGC 6251, concluding that electron scattering explains gamma-ray emission and setting limits on proton content.

Contribution

It provides a detailed spectral energy distribution analysis of these lobes, emphasizing the role of electron scattering and constraining proton contributions.

Findings

Gamma-ray emission explained by electron scattering

Upper bounds set on proton content in lobes

Analysis challenges hadronic origin hypotheses

Abstract

Radio and $\gamma$-ray measurements of large lobes of several radio galaxies provide adequate basis for determining whether emission in these widely separated spectral regions is largely by energetic electrons. This is very much of interest as there is of yet no unequivocal evidence for a significant energetic proton component to account for $\gamma$-ray emission by neutral pion decay. A quantitative assessment of the proton spectral distribution necessitates full accounting of the local and background radiation fields in the lobes; indeed, doing so in our recent analysis of the spectral energy distribution of the Fornax A lobes considerably weakened previous conclusions on the hadronic origin of the emission measured by the Fermi satellite. We present the results of similar analyses of the measured radio, X-ray and $\gamma$-ray emission from the lobes of Centaurus A, Centaurus B, and NGC 6251. The results indicate that the measured $\gamma$-ray emission from these lobes can be accounted for by Compton scattering of the radio-emitting electrons off the superposed radiation fields in the lobes; consequently, we set upper bounds on the energetic proton contents of the lobes.