Broadband Spectral Energy Distributions of Active Galactic Nuclei from an Accretion Disk with Advective Coronal Flow

摘要

Recent multiwaveband observations of Seyfert nuclei and QSOs established significant deviations in the spectral shape of the big blue bump from a blackbody spectral shape; soft X-ray excess has a spectral index α (Fν ∝ ν-α) of 1.6 and hard X-ray tail with α of ~0.7. We construct a disk-corona model which accounts for such broadband spectral properties. We study the 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 τc, 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 ~50 keV. The different spectral slopes (α ~ 1.5 below 2 keV and ~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 (≤300 Schwarzschild radii). The emergent optical spectrum is redder (α ~ 0.3) than that of the standard disk (α ~ -0.3), being consistent with observations, due to the different efficiencies of spectral hardening of disk emission at different radii. Further, we find that the cutoff frequency of the hard X-ray (~coronal electron temperature) and broadband 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 ∝M.