Modelling of the Spectral Energy Distribution of Fornax A: Leptonic and Hadronic Production of High Energy Emission from the Radio Lobes

TL;DR

This study models the spectral energy distribution of Fornax A, combining radio to gamma-ray data, and finds that X-ray emission is leptonic while gamma-ray emission is hadronic, revealing complex particle processes in radio galaxy lobes.

Contribution

It provides a comprehensive multi-wavelength analysis of Fornax A and proposes a novel interpretation of gamma-ray emission as hadronic, contrasting with standard leptonic models.

Findings

Radio lobes have similar spectral indices.

X-ray emission is consistent with inverse-Compton scattering.

Gamma-ray emission is likely due to hadronic processes.

Abstract

We present new low-frequency observations of the nearby radio galaxy Fornax A at 154 MHz with the Murchison Widefield Array, microwave flux-density measurements obtained from WMAP and Planck data, and gamma-ray flux densities obtained from Fermi data. We also compile a comprehensive list of previously published images and flux-density measurements at radio, microwave and X-ray energies. A detailed analysis of the spectrum of Fornax A between 154 MHz and 1510 MHz reveals that both radio lobes have a similar spatially-averaged spectral index, and that there exists a steep-spectrum bridge of diffuse emission between the lobes. Taking the spectral index of both lobes to be the same, we model the spectral energy distribution of Fornax A across an energy range spanning eighteen orders of magnitude, to investigate the origin of the X-ray and gamma-ray emission. A standard leptonic model for the production of both the X-rays and gamma-rays by inverse-Compton scattering does not fit the multi-wavelength observations. Our results best support a scenario where the X-rays are produced by inverse-Compton scattering and the gamma-rays are produced primarily by hadronic processes confined to the filamentary structures of the Fornax A lobes.