Abstract
We investigate a corona coupled with an underlying disk through the magnetic field and radiation field and present emergent spectra. As a result of 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 a given black hole mass and accretion rate, and then we 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 to 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 rates, i.e., ? ~ 1.1 (F? ?-?). 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 0.2LEdd) there exists only the solution of hard spectra, while for high-luminosity systems (L 0.8LEdd) there exist solutions of both hard and soft spectra. For moderate-luminosity systems (0.2LEdd L 0.8LEdd), 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 broadband spectra observed in narrow-line Seyfert 1 galaxies.
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