X-ray properties of intermediate-mass black holes in active galaxies. II. X-ray-bright accretion and possible evidence for slim disks

Louis Benoit Desroches, Jenny E. Greene, Luis C. Ho

Research output: Contribution to journalArticle

46 Scopus citations

Abstract

We present X-ray properties of optically selected intermediate-mass (∼105-106 M) black holes (BHs) in active galactic nuclei (AGNs), using data from the Chandra X-Ray Observatory. Our observations are a continuation of a pilot study by Greene & Ho. Of the eight objects observed, five are detected with X-ray luminosities in the range L0.5-2keV = 1041-1043 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 < Γs < 2.7, where N(E) ∝ E-Γs, 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 αox is flatter than in more massive systems, even other NLS1s. Only X-ray-selected NLS1s with very high Eddington ratios share a similar αox. 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.

Original languageEnglish (US)
Pages (from-to)1515-1522
Number of pages8
JournalAstrophysical Journal
Volume698
Issue number2
DOIs
StatePublished - Jun 20 2009

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Keywords

  • Galaxies: Seyfert
  • Galaxies: active
  • Galaxies: nuclei
  • Galaxies: statistics
  • X-rays: galaxies Online-only material: color figures

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