The Hot Disk Corona and Magnetic Turbulence in Radio-quiet Active Galactic Nuclei: Observational Constraints

Abstract

We compile a sample consisting of 56 radio-quiet active galactic nuclei so as to investigate statistical properties of hot corona of accretion disks from {\em ASCA} observations. The black-hole masses in the sample are estimated via several popular methods and the bolometric luminosities from the multi-wavelength continuum. This allows us to estimate the Eddington ratio (${\cal E}\equiv L_{\rm Bol}/L_{\rm Edd}$) so that the undergoing physical processes can be tested via hard X-ray data. We find a strong correlation between ${\cal F}_{\rm X}\equiv L_{\rm 2-10keV}/L_{\rm Bol}$ and ${\cal E}$ as ${\cal F}_{\rm X}\propto {\cal E}^{-0.64}$ with a multivariate regression. 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 ($\Gamma$) and ${\cal E}$ depends critically on the types of objects: $\Gamma$ is nearly constant ($\Gamma \propto {\cal E}^0$) in broad-line Seyfert 1's (BLS1s), whereas $\Gamma \propto \log {\cal E}^{0.18}$ in narrow-line Seyfert 1's (NLS1s), although not very significant. We can set constraints on the forms of magnetic stress tensor on the condition that ${\cal F}_{\rm X}$ is proportional to the fraction $f$ of gravitational energy dissipated in the hot corona and that $f$ is proportional to magnetic energy density in the disk. We find that the shear stress tensor $t_{r\phi}\propto P_{\rm gas}$ is favored by the correlation in the present sample, where $P_{\rm gas}$ is the gas pressure.