Weakly magnetized black holes as particle accelerators

We study the collision of particles in the vicinity of a horizon of a weakly magnetized nonrotating black hole. In the presence of the magnetic field innermost stable circular orbits of charged particles can be located close to the horizon. We demonstrate that for a collision of two particles, one of which is charged and revolving at innermost stable circular orbits and the other is neutral and falling from infinity, the maximal collision energy can be high in the limit of a strong magnetic field. This effect has some similarity with the recently discussed effect of high center-of-mass energy for the collision of particles in extremely rotating black holes. We also demonstrate that for ``realistic'' astrophysical black holes their ability to play the role of ``accelerators'' is in fact quite restricted.