ACCELERATION OF PARTICLES AS A UNIVERSAL PROPERTY OF ERGOSPHERE

We show that recent observation made by Grib and Pavlov, [A. A. Grib and Yu. V. Pavlov, Europhys. Lett. 101 , 20004 (2013)] for the Kerr black hole is valid in the general case of rotating axially symmetric metric. Namely, collision of two particles in the ergosphere leads to indefinite growth of the energy in the center-of-mass frame, provided the angular momentum of one of the two particles is negative and increases without limit for a fixed energy at infinity. General approach enabled us to elucidate why the role of the ergosphere is crucial in this process.