  We present a model for high-energy emission sources generated by a standing
magnetohydrodynamical (MHD) shock in a black hole magnetosphere. The black hole
magnetosphere would be constructed around a black hole with an accretion disk,
where a global magnetic field could be originated by currents in the accretion
disk and its corona. Such a black hole magnetosphere may be considered as a
model for the central engine of active galactic nuclei, some compact X-ray
sources and gamma-ray bursts. The energy sources of the emission from the
magnetosphere are the gravitational and electromagnetic energies of magnetized
accreting matters and the rotational energy of a rotating black hole. When the
MHD shock generates in MHD accretion flows onto the black hole, the plasma's
kinetic energy and hole's rotational energy can convert to radiative energy. In
this letter, we demonstrate the huge energy output at the shock front by
showing negative energy postshock accreting MHD flows for a rapidly rotating
black hole. This means that the extracted energy from the black hole can
convert to the radiative energy at the MHD shock front. When axisymmetric shock
front is formed, we expect a ring-shaped region with very hot plasma near the
black hole; the look would be like an "aurora". The high energy radiation
generated from there would carry to us the information for the curved spacetime
due to the strong gravity.
