paper

X-ray Properties of Intermediate-Mass Black Holes in Active Galaxies. II. X-ray-Bright Accretion and Possible Evidence for Slim Disks

arXiv:0903.2257 · doi:10.1088/0004-637X/698/2/1515

Abstract

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 < Γ_s < 2.7, where N(E) \propto E^-Γ_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 α_ox is flatter than in more massive systems, even other NLS1s. Only X-ray-selected NLS1s with very high Eddington ratios share a similar α_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.

11 pages, 6 figures, submitted to ApJ, emulateapj

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