TIME-VARIATION OF MAGNETOHYDRODYNAMIC ACCRETION ONTO A BLACK-HOLE

The non-stationary and non-axisymmetric perturbations of magnetohydrod ynamic (MHD) accretion onto a Kerr black hole are studied. We regard a small-amplitude fluctuation of the electromagnetic field as being a s ource of perturbations in a magnetosphere and examine how the plasma a ccretion is affected by it along a flow line, especially at the fast-m agnetosonic point in the short-wavelength limit. Assuming that the mag netic field dominates the plasma accretion, we find that the meridiona l current becomes large near to the fast-magnetosonic point located cl ose to the horizon. This is due to the effects of the particle's inert ia and the strong gravity near the horizon. As a consequence, the flui d suffers a large radial acceleration resulting from the Lorentz force , and becomes highly variable compared with the electromagnetic field there. This variable nature of the plasma accretion may be responsible for the variability of the higher energy X-rays observed from a galac tic black hole candidate (GBHC) or an active galactic nucleus (AGN).