We study numerically particle acceleration by the electric field induced near the horizon of a rotating supermassive (M \sim 10^9-10^{ 10} M_{ \odot}) black hole embedded in the magnetic field B. We find that acceleration of protons to energy E > 10^{ 20} eV is possible only at extreme values of M and B. We also find that the acceleration is very inefficient and is accompanied by a broad band MeV-TeV radiation whose total power exceeds at least by a factor of 1000 the total power emitted in ultra-high energy cosmic rays (UHECR). This implies that if O(10) nearby quasar remnants were sources of proton events with energy E > 10^{ 20} eV, then each quasar remnant would overshine e.g. the Crab nebula by more than two orders of magnitude in the TeV energy band. Recent TeV observations exclude this possibility. A model in which O(100) sources are situated at 100-1000 Mpc is not ruled out and can be experimentally tested by present TeV gamma-ray telescopes. Such a model can explain the observed UHECR flux at moderate energies E \sim (4-5) 10^{ 19} eV.
Categories: astro-ph hep-th