2018ApJ...867..120H

We investigate the electron-positron pair cascade taking place in the magnetosphere of a rapidly rotating black hole. Because of the spacetime frame dragging, the Goldreich-Julian charge density changes sign in the vicinity of the event horizon, which leads to the occurrence of a magnetic-field-aligned electric field, in the same way as the pulsar outer-magnetospheric accelerator. In this lepton accelerator, electrons and positrons are accelerated in the opposite directions, to emit copious gamma rays via the curvature and inverse Compton processes. We examine a stationary pair cascade and show that a stellar-mass black hole moving in a gaseous cloud can emit a detectable very high energy flux, provided that the black hole is extremely rotating and that the distance is less than about 1 kpc. We argue that the gamma-ray image will have a point-like morphology, and we demonstrate that their gamma-ray spectra have a broad peak around 0.01-1 GeV and a sharp peak around 0.1 TeV, that the accelerators become most luminous when the mass accretion rate becomes about 0.01% of the Eddington rate, and that the predicted gamma-ray flux changes little in a wide range of magnetospheric currents. An implication of the stability of such a stationary gap is discussed.