X-ray Properties of Intermediate-Mass Black Holes in Active Galaxies.
  II. X-ray-Bright Accretion and Possible Evidence for Slim Disks
  We present X-ray properties of optically-selected intermediate-mass
(~10^5--10^6 M_Sun) black holes (BHs) in active galaxies (AGNs), using data
from the Chandra X-Ray Observatory. Our observations are a continuation of a
pilot study by Greene & Ho (2007). Of the 8 objects observed, 5 are detected
with X-ray luminosities in the range L_0.5-2 keV = 10^41--10^43 erg s^-1,
consistent with the previously observed sample. Objects with enough counts to
extract a spectrum are well fit by an absorbed power law. We continue to find a
range of soft photon indices 1 < \Gamma_s < 2.7, where N(E) \propto
E^-\Gamma_s, consistent with previous AGN studies, but generally flatter than
other narrow-line Seyfert 1 active nuclei (NLS1s). The soft photon index
correlates strongly with X-ray luminosity and Eddington ratio, but does not
depend on BH mass. There is no justification for the inclusion of any
additional components, such as a soft excess, although this may be a function
of the relative inefficiency of detecting counts above 2 keV in these
relatively shallow observations. As a whole, the X-ray-to-optical spectral
slope \alpha_ox is flatter than in more massive systems, even other NLS1s. Only
X-ray-selected NLS1s with very high Eddington ratios share a similar \alpha_ox.
This is suggestive of a physical change in the accretion structure at low
masses and at very high accretion rates, possibly due to the onset of slim
disks. Although the detailed physical explanation for the X-ray loudness of
these intermediate-mass BHs is not certain, it is very striking that targets
selected on the basis of optical properties should be so distinctly offset in
their broader spectral energy distributions.
