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Bañados-Silk-West effect with nongeodesic particles: extremal horizons

The Bañados-Silk-West (BSW) effect consists in the possibility to obtain arbitrarily large energy $E_{c.m.}$ in the centre of mass frame of two colliding particles near the black hole horizon. One of the common beliefs was that the action of force on these particles (say, due to gravitational radiation) should necessarily restrict the growth of $E_{c.m.}$. We consider extremal horizons and develop a model-independent approach and analyze the conditions for the force to preserve or kill the effect, using the frames attached both to observers orbiting the black hole and to ones crossing the horizon. We argue that the aforementioned expectations are not confirmed. Under rather general assumptions, the BSW effect survives. For equatorial motion it is only required that in the proper frame the radial component of the force be finite, while the azimuthal one tend to zero not too slowly. If the latter condition is violated, we evaluate $E_{c.m.}$, which becomes indeed restricted but remains very large for small forces.