Radio Quiet AGNs as Possible Sources of Ultra-high Energy Cosmic Rays

TL;DR

This paper proposes that radio quiet AGNs could be sources of ultra-high-energy cosmic rays if heavy nuclei are involved, highlighting the potential for acceleration in weak jets and discussing observational implications.

Contribution

It demonstrates that radio quiet AGNs can accelerate heavy nuclei to ultra-high energies within parsec-scale jets, expanding potential UHECR sources beyond radio loud AGNs.

Findings

Heavy nuclei can survive photodisintegration in weak jets.

Acceleration to 10^{20} eV is feasible in parsec-scale jets.

Energy production rate per source is compatible with observed UHECR flux.

Abstract

Active galactic nuclei (AGNs) have been one of the most widely discussed sources of ultrahigh-energy cosmic rays (UHECRs). The recent results of Pierre Auger observatory (PAO) have indicated a possible composition change of UHECRs above ~10^{18.5} eV towards heavy nuclei. We show here that if indeed UHECRs are largely heavy nuclei, then nearby radio quiet AGNs can also be viable sources of UHECRs. We derive constraints on the acceleration sites which enable acceleration of UHECRs to 10^{20} eV without suffering losses. We show that the acceleration of UHECRs and the survival of energetic heavy nuclei are possible in the parsec scale weak jets that are typically observed in these objects, the main energy loss channel being photodisintegration. On this scale, energy dissipation by shock waves resulting from interactions inside a jet or of the jet with surrounding material are expected, which may accelerate the particles up to very high energies. We discuss the possible contribution of radio-quiet AGNs to the observed UHECR flux, and show that the required energy production rate in UHECRs by a single object could be as low as ~ 3*10^{39} erg/s, which is less than a percent of the bolometric luminosity, and thus energetically consistent. We discuss consequences of this model, the main one being the difficulty in detecting energetic secondaries (\gamma-rays and neutrinos) from the same sources.