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