Ultra-high energy collisions of nonequatorial geodesic particles near   dirty black holes
  We consider collision of two geodesic particles moving around rotating stationary axially symmetric black holes. It is shown for arbitrary nonequatorial motion that under certain conditions the energy in their centre of mass frame can grow unbound (the so-called BSW effect). This generalizes the previous results for equatorial motion around dirty (surrounded by matter) black holes and nonequatorial motion around the Kerr metric. It turns out that the BSW effect occurs near any point of the horizon surface. We do not use special symmetries of space-time typical of the Kerr metric, so the results are quite generic. The general scheme classifying all possible scenarios is discussed. 
O. B. Zaslavskii
gr-qc astro-ph.HE hep-th