  Production and acceleration mechanisms of ultra-high-energy cosmic rays
(UHECRs) of energy $>10^{20}$eV, clearly beyond the GZK-cutoff limit remain
unclear that points to exotic nature of the phenomena. Recent observations of
extragalactic neutrino may indicate the source of UHECRs being an extragalactic
supermassive black hole (SMBH). We demonstrate that ultra-efficient energy
extraction from rotating SMBH driven by the magnetic Penrose process (MPP)
could indeed foot 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 $10^{20}$eV for SMBH of mass $10^9 M_{\odot}$ and magnetic
field of strength $10^4$G. Applied to Galactic center SMBH we have proton
energy of order $\approx 10^{15.6}$eV that coincides with the knee of the
cosmic ray spectra. We show that large $\gamma_z$ factors of high-energy
particles along the escaping directions occur only in the presence of induced
charge of the black hole that is known as the Wald charge in the case of
uniform magnetic field. It is remarkable that the process neither requires
extended acceleration zone, nor fine-tuning of accreting matter parameters.
Further, this leads to certain verifiable constraints on SMBH's mass and
magnetic field strength as UHECRs sources. This clearly makes ultra-efficient
regime of MPP one of the most promising mechanisms for fueling UHECRs
powerhouse.
