Supermassive black holes as possible sources of ultra high energy cosmic rays
Abstract
Production and acceleration mechanisms of ultra-high-energy cosmic rays (UHECRs) of energy >1020eV, 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 1020eV for SMBH of mass 109 M and magnetic field of strength 104G. Applied to Galactic center SMBH we have proton energy of order ≈ 1015.6eV that coincides with the knee of the cosmic ray spectra. We show that large γz 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.