Ultrahigh energy particle collisions near the black hole horizon in the   strong magnetic field
  We consider collision between two charged (or charged and neutral) particles near the black hole horizon in the strong magnetic field B. It is shown that there exists a strip near the horizon within which collision of any two such particles leads to ultrahigh energy in the centre of mass frame. The results apply to generic (not necessarily vacuum) black holes. 
O. B. Zaslavskii
gr-qc astro-ph.HE hep-th