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Supermassive black holes are believed to be the central power house of activegalactic nuclei. Applying the pulsar outer-magnetospheric particle acceleratortheory to black-hole magnetospheres, we demonstrate that an electric field isexerted along the magnetic field lines near the event horizon of a rotatingblack hole. In this particle accelerator (or a gap), electrons and positronsare created by photon-photon collisions and accelerated in the oppositedirections by this electric field, efficiently emitting gamma-rays viacurvature and inverse-Compton processes. It is shown that a gap arises aroundthe null charge surface formed by the frame-dragging effect, provided thatthere is no current injection across the gap boundaries. The gap is dissipatinga part of the hole's rotational energy, and the resultant gamma-ray luminosityincreases with decreasing plasma accretion from the surroundings. Consideringan extremely rotating supermassive black hole, we show that such a gapreproduces the significant very-high-energy (VHE) gamma-ray flux observed fromthe radio galaxy IC 310, provided that the accretion rate becomes much lessthan the Eddington rate particularly during its flare phase. It is found thatthe curvature process dominates the inverse-Compton process in themagnetosphere of IC~310, and that the observed power-law-like spectrum in VHEgamma-rays can be explained to some extent by a superposition of the curvatureemissions with varying curvature radius. It is predicted that the VHE spectrumextends into higher energies with increasing VHE photon flux.

Energetic Gamma Radiation from Rapidly Rotating Black Holes