Circular orbits and acceleration of particles by near-extremal dirty   rotating black holes: general approach
  We study the effect of ultra-high energy particles collisions near the black hole horizon (BSW effect) for two scenarios: when one of particle either (i) moves on a circular orbit or (ii) plunges from it towards the horizon. It is shown that such circular near-horizon orbits can exist for near-extremal black holes only. This includes the innermost stable orbit (ISCO), marginally bound orbit (MBO) and photon one (PhO). We consider generic "dirty" rotating black holes not specifying the metric and show that the energy in the centre of mass frame has the universal scaling dependence on the surface gravity $\kappa $. Namely, $E_{c.m.}\sim \kappa ^{-n}$ where for the ISCO $n=1/3$ in case (i) or $n=1/2$ in case (ii). For the MBO and PhCO $n=1/2$ in both scenarios that agrees with recent calculations of Harada and Kimura for the Kerr metric. We also generalize the Grib and Pavlov's observations made for the Kerr metric. The magnitude of the BSW effect on the location of collision has a somewhat paradoxical character: it is decreasing when approaching the horizon. 
O. B. Zaslavskii
gr-qc astro-ph.HE hep-th