Recent multi-waveband observations of Seyfert nuclei and QSOs established significant deviations in the spectral shape of the big blue bump from a blackbody one ; Soft X-ray excess has a spectral index \alpha ( { F _ { \nu } } \propto \nu ^ { - \alpha } ) of 1.6 and hard X-ray tail with \alpha of \sim 0.7 .
We construct a disk-corona model which accounts for such broad-band spectral properties .
We study emission spectrum emerging from a vertical disk-corona structure composed of two-temperature plasma by solving hydrostatic equilibrium and radiative transfer self-consistently .
A fraction f of viscous heating due to mass accretion is assumed to be dissipated in a corona with a Thomson optical depth of { \tau _ { c } } , where advective cooling is also included , and a remaining fraction , 1 - f , dissipates within a main body of the disk .
Our model can nicely reproduce the soft X-ray excess with a power-law shape and the hard tail extending to \sim 50 keV .
The different spectral slopes ( \alpha \sim 1.5 below 2keV and \sim 0.5 above ) are the results of different emission mechanisms and different sites ; the former slope is due to unsaturated Comptonization from the innermost zone and the latter is due to a combination of the Comptonization , bremsstrahlung and a reflection of the coronal radiation at the disk-corona boundary from the inner to surrounding zone ( \leq 300 Schwarzschild radii ) .
The emergent optical spectrum is redder ( \alpha \sim 0.3 ) than that of the standard disk ( \alpha \sim - 0.3 ) , being consistent with observations , due to the different efficiencies of spectral distortion of disk emission at different radii .
Further , we find that the cut-off frequency of the hard X-ray ( \sim coronal electron temperature ) and broad-band spectral shape are insensitive to the black-hole mass , while the peak frequency of the big blue bump is sensitive to the mass as the peak frequency \propto { M _ { BH } } ^ { -1 / 4 } .