We investigate a corona coupled with underlying disk through magnetic field and radiation field , and present emergent spectra .
Due to buoyancy the magnetic flux loop emerges from the disk and reconnects with other loops in the corona , thereby releasing the magnetic energy to heat the coronal plasma .
The energy is then radiated away through Compton scattering .
By studying the energy balance in the corona , transition layer and disk , we determine the fraction ( f ) of accretion energy dissipated in the corona for given black-hole mass and accretion rate , and then determine the coronal and disk variables .
This allows us to calculate emergent spectra through Monte Carlo simulations .
The spectra are then determined as functions of 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 the same for different accretion rate , i.e . \alpha \sim 1.1 ( F _ { \nu } \propto \nu ^ { - \alpha } ) .
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 peaked in UV and soft X-rays .
For low-luminosity systems ( L \lesssim 0.2 L _ { Edd } ) , there exists only the solution of hard spectra ; While for high-luminosity systems ( L \gtrsim 0.8 L _ { Edd } ) , there exist both solutions of hard and soft spectra .
For middle-luminosity systems ( 0.2 L _ { Edd } \lesssim L \lesssim 0.8 L _ { Edd } ) , 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 .