  We investigate a corona coupled with underlying disk through magnetic field
and radiation field, and present emergent spectra calculated from Monte Carlo
simulations. The spectra are determined for given black-hole mass and accretion
rate. We find two types of solutions corresponding for hard spectrum and soft
spectrum. In the hard-spectrum solution, the accretion energy is dominantly
dissipated in the corona, supporting a strong corona above a cool disk; The
hard X-ray spectral indices are around 1.1, same for different accretion rates.
In the soft-spectrum solution, the accretion energy is mainly dissipated in the
disk. The coronal temperature and density are quite low. Consequently, the
spectra are dominated by the disk radiation peaking at UV and soft X-rays. For
low-luminosity systems, there exists only the solution of hard spectra; While
for high-luminosity systems, there exist both solutions of hard and soft
spectra. For middle-luminosity systems, besides the hard spectra, moderately
soft spectra composed of an inner soft-spectrum solution and an outer
hard-spectrum solution may occur, the softness of which increases with
increasing luminosity. The hard spectra are close to the observed spectra in
Seyfert galaxies and radio-quiet QSOs. The composite spectra may account for
the diversity of broad band spectra observed in narrow-line Seyfert 1 galaxies.
