A Correlation Between the Ionization State of the Inner Accretion Disk and the Eddington Ratio of Active Galactic Nuclei

Abstract: X-ray reflection features observed from the innermost regions of accretion disks in Active Galactic Nuclei (AGNs) allow important tests of accretion theory. In recent years it has been possible to use the Fe K line and reflection continuum to parametrize the ionization state of the irradiated inner accretion disk. Here, we collect 10 measurements of xi, the disk ionization parameter, from 8 AGNs with strong evidence for reflection from the inner accretion disk and good black hole mass estimates. We find strong statistical evidence (98.56% confidence) for a nearly linear correlation between xi and the AGN Eddington ratio. Moreover, such a correlation is predicted by a simple application of alpha-disk accretion theory, albeit with a stronger dependence on the Eddington ratio. The theory shows that there will be intrinsic scatter to any correlation as a result of different black hole spins and radii of reflection. There are several possibilities to soften the predicted dependence on the Eddington ratio to allow a closer agreement with the observed correlation, but the current data does not allow for an unique explanation. The correlation can be used to estimate that MCG-6-30-15 should have a highly ionized inner accretion disk, which would imply a black hole spin of ~0.8. Additional measurements of xi from a larger sample of AGNs are needed to confirm the existence of this correlation, and will allow investigation of the accretion disk/corona interaction in the inner regions of accretion disks.