COLLIMATION OF HIGHLY VARIABLE MAGNETOHYDRODYNAMIC DISTURBANCES AROUND A ROTATING BLACK-HOLE

We have studied nonstationary and nonaxisymmetric perturbations of a m agnetohydrodynamic accretion onto a rotating (Kerr) black hole. Assumi ng that the magnetic field dominates the plasma accretion, we find tha t the accretion suffers a large radial acceleration resulting from the Lorentz force and becomes highly variable compared with the electroma gnetic field on the rotating black hole. In fact, we further find an i nteresting perturbed structure of the plasma velocity with a large pea k in some narrow region located slightly inside of the fast-magnetoson ic surface. This is due to the concentrated propagation of the fluid d isturbances in the form of fast-magnetosonic waves along the separatri x surface. If the fast-magnetosonic speed is smaller in the polar regi ons than in the equatorial regions, the critical surface has a prolate shape for radial poloidal field lines. In this case, only the waves t hat propagate toward the equator can escape from the super-fast-magnet osonic region and collimate poleward as they propagate outward in the sub-fast-magnetosonic regions. We further discuss the capabilities of such collimated waves in accelerating particles due to cyclotron reson ance in an electron-positron plasma.