Broadband spectral energy distributions of active galactic nuclei from an accretion disk with advective coronal flow

Recent multiwaveband observations of Seyfert nuclei and QSOs established si gnificant deviations in the spectral shape of the big blue bump from a blac kbody spectral shape; soft X-ray excess has a spectral index alpha (F-nu pr oportional to nu (-alpha)) of 1.6 and hard X-ray tail with alpha of similar to0.7. We construct a disk-corona model which accounts for such broadband spectral properties. We study the emission spectrum emerging from a vertica l disk-corona structure composed of two-temperature plasma by solving hydro static equilibrium and radiative transfer self-consistently. A fraction f o f viscous heating due to mass accretion is assumed to be dissipated in a co rona with a Thomson optical depth of where advective cooling is also includ ed, and a remaining fraction, 1 - f, dissipates within a main body of the d isk. Our model can nicely reproduce the soft X-ray excess with a power-law shape and the hard tail extending to similar to 50 keV. The different spect ral slopes (alpha similar to 1.5 below 2 keV and similar to0.5 above) are t he results of different emission mechanisms and different sites; the former slope is due to unsaturated Comptonization from the innermost zone, and th e 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 (less than or equal to 300 Schwarzschild radii). The emergent optical spectrum is redder (alpha similar to 0.3) than that o f the standard disk (alpha similar to -0.3), being consistent with observat ions, due to the different efficiencies of spectral hardening of disk emiss ion at different radii. Further, we find that the cutoff frequency of the h ard X-ray (similar to coronal electron temperature) and broadband spectral shape are insensitive to the black hole mass, while the peak frequency of t he big blue bump is sensitive to the mass as the peak frequency proportiona l to M-BH(1/4).