Radio-quiet active galactic nuclei as possible sources of ultrahigh-energy cosmic rays

Active galactic nuclei (AGNs) have been one of the most widely discussed sources of ultrahigh-energy cosmic rays (UHECRs). The recent results of the Pierre Auger observatory have indicated a possible composition change of UHECRs above ~1018.5 eV toward 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 10²⁰ 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 ≈3×10³⁹ erg s^-1, 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 (γ-rays and neutrinos) from the same sources.