The origin of the very-high-energy radiation along the jet of Centaurus A

TL;DR

This study investigates whether stationary knots in Centaurus A's jet, formed by jet-star interactions, are the sites of particle acceleration responsible for observed very-high-energy gamma-ray emission.

Contribution

It combines hydrodynamic simulations and shock acceleration theory to link jet knots with VHE gamma-ray production in Centaurus A.

Findings

Knots can accelerate electrons to energies producing VHE gamma rays.

Electrons at knots explain the observed gamma-ray spectrum.

Jet-star interactions are plausible acceleration sites.

Abstract

As the closest known active galactic nucleus, Centaurus A (Cen A) provides a rich environment for astrophysical exploration. It has been observed across wavelengths from radio to gamma rays, and indications of ongoing particle acceleration have been found on different scales. Recent measurements of very-high-energy (VHE) gamma-rays ($>240$ GeV) by the HESS observatory have inferred the presence of ultra-relativistic electrons along Cen A's jet, yet the underlying acceleration mechanism remains uncertain. Various authors have proposed that jet substructures, known as knots, may serve as efficient particle accelerators. In this study, we investigate the hypothesis that knots are the particle acceleration sites along Cen A's jets. We focus on stationary knots, and assume that they result from interactions between the jet and the stellar winds of powerful stars. By combining relativistic hydrodynamic simulations and shock acceleration theory with the radio and X-ray data, we compare theoretical predictions with morphological and spectral data from different knots. We estimate the maximum electron energy and the resulting VHE gamma-ray emission. Our findings suggest that electrons accelerated at the knots are responsible for the gamma-ray spectrum detected in the VHE band.