An accretion disc-corona model for X-ray spectra of active galactic nuclei

The hard X-ray emission of active galactic nuclei (AGN) is believed to originate from the hot coronae above the cold accretion discs. The hard X-ray spectral index is found to be correlated with the Eddington ratio L-bol/L-Edd, and the hard X-ray bolometric correction factor L-bol/L-X,L-2-10keV increases with the Eddington ratio. The Compton reflection is also found to be correlated with the hard X-ray spectral index for Seyfert galaxies and X-ray binaries. These observational features provide very useful constraints on the accretion disc-corona model for AGN. We construct an accretion disc-corona model and calculate the spectra with different magnetic stress tensors in the cold discs, in which the corona is assumed to be heated by the reconnection of the magnetic fields generated by buoyancy instability in the cold accretion disc. Our calculations show that the magnetic stress tensor tau(r phi) = alpha p(gas) fails to explain all these observational features, while the disc-corona model with tau(r phi) = alpha p(tot) always leads to constant L-bol/L-X,L-2-10keV independent of the Eddington ratio. The resulted spectra of the disc-corona systems with tau(r phi) = alpha root p(gas)p(tot) show that both the hard X-ray spectral index and the hard X-ray bolometric correction factor L-bol/L-X,L-2-10keV increase with the Eddington ratio, which are qualitatively consistent with the observations. We find that the disc-corona model is unable to reproduce the observed very hard X-ray continuum emission from the sources accreting at low rates (e.g. Gamma similar to 1 for L-bol/L-Edd similar to 0.01), which may imply the different accretion modes in these low-luminosity sources. We suggest that the disc-corona system transits to an advection-dominated accretion flow+disc corona system at low accretion rates, which may be able to explain all the above-mentioned correlations.