X-ray Properties of Intermediate-mass Black Holes in Active Galaxies.
  III. Spectral Energy Distribution and Possible Evidence for Intrinsically
  X-ray-weak AGNs
  We present a systematic X-ray study, the third in a series, of 49 active
galactic nuclei with intermediate-mass black holes (IMBH; ~10^5-10^6 M_sun)
using Chandra observations. We detect 42 out of 49 targets with a 0.5-2 keV
X-ray luminosity 10^41-10^43 erg/s. We perform spectral fitting for the 10
objects with enough counts (>200), and they are all well fit by a simple
power-law model modified by Galactic absorption, with no sign of significant
intrinsic absorption. While we cannot fit the X-ray spectral slope directly for
the rest of the sample, we estimate it from the hardness ratio and find a range
of photon indices consistent with those seen in more luminous and massive
objects. The X-ray-to-optical spectral slope (alphaox) of our IMBH sample is
systematically flatter than in active galaxies with more massive black holes,
consistent with the well-known correlation between alphaox and UV luminosity.
Thanks to the wide dynamic range of our sample, we find evidence that alphaox
increases with decreasing M_BH as expected from accretion disk models, where
the UV emission systematically decreases as M_BH decreases and the disk
temperature increases. We also find a long tail toward low alphaox values.
While some of these sources may be obscured, given the high L_bol/L_Eddington
values in the sample, we argue that some may be intrinsically X-ray-weak,
perhaps owing to a rare state that radiates very little coronal emission.
