2004ApJ...607L.107W

We compile a sample consisting of 56 radio-quiet active galactic nuclei so as to investigate the statistical properties of the hot corona in accretion disks from ASCA observations. The black hole masses in the sample are estimated via several popular methods, and the bolometric luminosities from the multiwavelength continuum. This allows us to estimate the Eddington ratio (E≡L_bol/L_Edd) so that the undergoing physical processes can be tested via hard X-ray data. We use the least-squares method of multivariate regression and find a strong correlation between F_X≡L_2–10keV/L_bol and E as F_X∝E^–0.64. This indicates that the release of gravitational energy in the hot corona is controlled by the Eddington ratio. On the other hand, the correlation between the hard X-ray spectral index (Γ) and E depends critically on the types of objects: Γ is nearly constant (Γ∝E⁰) in broad-line Seyfert 1 galaxies, whereas Γ∝logE^0.18 in narrow-line Seyfert 1 galaxies, although this is not very significant. We can set constraints on the forms of the magnetic stress tensor on the condition that F_X is proportional to the fraction f of gravitational energy dissipated in the hot corona and that f is proportional to the magnetic energy density in the disk. We find that the shear stress tensor t_rφ∝P_gas is favored by the correlation in the present sample, where P_gasis the gas pressure.