High energy collision of particles in the magnetic field far from black   holes
  We consider collision of two particles in the axially symmetric black hole metric in the magnetic field. If the value of the angular momentum |L| of one particles grows unbound (but its Killing energy remains fixed) one can achieve unbound energy in the center of mass frame E_{c.m.} In the absence of the magnetic field, collision of this kind is known to happen in the ergoregion. However, if the magnetic field strength B is also large, with the ratio |L|/B being finite, large E_{c.m.} can be achieved even far from a black hole, in the almost flat region. Such an effect also occurs in the metric of a rotating star. 
O. B. Zaslavskii
gr-qc astro-ph.HE hep-th