Magnetic Stress at the Marginally Stable Orbit: Altered Disk Structure, Radiation, and Black Hole Spin Evolution

Abstract

Magnetic connections to the plunging region can exert stresses on the inner edge of an accretion disk around a black hole. We recompute the relativistic corrections to the thin-disk dynamics equations when these stresses take the form of a time-steady torque on the inner edge of the disk. The additional dissipation associated with these stresses is concentrated relatively close outside the marginally stable orbit, scaling as r-7/2 at large radius. As a result of these additional stresses, spin-up of the central black hole is retarded; the maximum spin-equilibrium accretion efficiency is 36% and occurs at a/M = 0.94; the disk spectrum is extended toward higher frequencies; line profiles (such as Fe Kα) are broadened if the line emissivity scales with local flux; limb brightening, especially at the higher frequencies, is enhanced; and the returning radiation fraction is substantially increased, up to 58%. This last effect creates possible explanations for both synchronized continuum fluctuations in active galactic nuclei and polarization rises shortward of the Lyman edge in quasars. We show that no matter what additional stresses occur, when a/M < 0.36, the second law of black hole dynamics sets an absolute upper bound on the accretion efficiency.