2020ApJ...895...14T

The production and acceleration mechanisms of ultrahigh-energy cosmic rays (UHECRs) of energy >10²⁰ eV, clearly beyond the GZK cutoff limit, remain unclear, which points to the exotic nature of the phenomena. Recent observations of extragalactic neutrinos may indicate that the source of UHECRs is an extragalactic supermassive black hole (SMBH). We demonstrate that ultraefficient energy extraction from a rotating SMBH driven by the magnetic Penrose process (MPP) could indeed fit the bill. We envision ionization of neutral particles, such as neutron beta decay, skirting close to the black hole horizon that energizes protons to over 10²⁰ eV for an SMBH of mass 10⁹M_☉ and magnetic field 10⁴ G. Applied to the Galactic center SMBH, we have a proton energy of order ≃10^15.6 eV that coincides with the knee of the cosmic-ray spectra. We show that large γ_z factors of high-energy particles along the escaping directions occur only in the presence of an induced charge of the black hole, which is known as the Wald charge in the case of a uniform magnetic field. It is remarkable that the process requires neither an extended acceleration zone nor fine-tuning of accreting-matter parameters. Further, this leads to certain verifiable constraints on the SMBH's mass and magnetic field strength as the source of UHECRs. This clearly makes the ultraefficient regime of the MPP one of the most promising mechanisms for fueling the UHECR powerhouse.