Abstract
We investigate a corona coupled with an underlying disk through magnetic and radiation fields. By studying the interaction between the corona and disk, we determine the fraction of accretion energy dissipated in the corona for a given black-hole mass and accretion rate. This allows us to calculate the 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 to hard spectrum and soft spectrum. In the hard-spectrum solution, the accretion energy is dominantly dissipated in the corona. The hard X-ray spectral indices are the same for different accretion rate, i.e., α ∼ 1.1. In the soft-spectrum solution, the accretion energy is mainly dissipated in the disk. The spectra are dominated by the disk radiation peaked in UV and soft X-rays. For low-luminosity systems ( L ⩽ 0.2 L Edd), there exists only the solution of hard spectra; while for high-luminosity systems ( L ⩾ 0.8 L Edd), there exist both solutions of hard and soft spectra. The hard spectra are close to the observed spectra in Seyfert galaxies and radio-quiet QSOs.
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