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

TL;DR

This study investigates X-ray emissions from intermediate-mass black holes in active galaxies, revealing a correlation between X-ray properties and accretion rates, and suggesting the possible presence of slim disks at high accretion levels.

Contribution

It provides new X-ray observational data on intermediate-mass black holes, highlighting differences in spectral energy distributions and proposing a physical change in accretion structures at low masses.

Findings

X-ray luminosities range from 10^41 to 10^43 erg s^-1.

Soft photon indices vary from 1 to 2.7, flatter than typical NLS1s.

Flatter x in these objects suggests a different accretion regime.

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 < \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.