Electromagnetic power induced from pair plasma falling into a rotating black hole II: An extensive WKB analysis in the slow rotation case

Abstract: We examine Poynting flux generation due to pair plasma accreting onto a slowly rotating black hole. In particular, we consider the possibility of an outgoing flux at the horizon. Our approach is based on a two-fluid model representing a collisionless pair plasma. In the background, the plasma inflow is neutral and radial along the magnetic field lines of a split monopole in a Schwarzschild spacetime. A combined mechanism of dragging by the black hole's spin and the Lorentz force produces charge separation and current flow, and hence electric and toroidal magnetic fields. By WKB analysis, two classes of solutions of perturbation equations for small black hole spin are identified: one is related to inward flux, and the amplitude inwardly increases; the other generates an outward flux with a peak position around $r=3M$, tending to zero at the horizon. The power induced by the two-fluid effect is inversely proportional to plasma density, and is small in almost all astrophysical situations. A magnetic vacuum region is located elsewhere for effective Poynting flux generation.