The Production of Ultra High Energy Cosmic Rays during the Early Epochs of Radio-loud AGN

TL;DR

This paper investigates the potential of young radio-loud AGNs, specifically compact symmetric objects, to accelerate protons to ultra-high energies and contribute to cosmic rays, highlighting their limitations and possible compositions.

Contribution

It demonstrates that compact symmetric objects can accelerate protons to ultra-high energies via Fermi mechanisms, but their contribution to cosmic rays is limited by their number density.

Findings

CSOs can accelerate protons up to 10^{20} eV at hot spots.

The local density of CSOs is insufficient to explain all UHECRs.

Heavy nuclei may be more feasible for UHECR acceleration in CSOs.

Abstract

Powerful radio-loud active galactic nuclei (AGN) with large Mpc-scale jets have been theoretically motivated as emitters of high-energy cosmic rays. Recent radio observations have established a populous class of young radio-loud galaxies with compact ($< 1$ kpc) symmetric jets that are morphologically similar to large-scale AGNs. We show that these compact AGNs, so-called compact symmetric objects (CSOs), can accelerate protons up to $10^{20}$ eV at their hot spots via a Fermi type mechanism on the assumption of efficient acceleration. The required magnetic field strengths are comparable to those derived from the minimum energy condition. We further show that the accelerated protons can escape through the photon fields of the cocoon without significant energy loss. However, the local number density of powerful CSOs is insufficient for CSOs to power the entire observed flux of ultra-high-energy cosmic rays, providing maximally only a few percent. A heavy composition of UHECRs allows more CSOs to accelerate particles to UHECR energies, but escaping the cocoon is difficult. We comment on a method that may test CSOs as UHECR sources.