TY - JOUR
T1 - X-ray properties of intermediate-mass black holes in active galaxies. II. X-ray-bright accretion and possible evidence for slim disks
AU - Desroches, Louis Benoit
AU - Greene, Jenny E.
AU - Ho, Luis C.
N1 - Funding Information:
We thank an anonymous referee for thoughtful comments. L.-B.D. thanks Eliot Quataert and J.E.G. Peek for helpful and insightful discussions. Support for this work was provided by NASA through grant SAO 08700135 issued by the Chandra Xray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory on behalf of NASA under contract NAS8-03060, and through grant HST-GO-11130.01-A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555.
Funding Information:
We thank an anonymous referee for thoughtful comments. L.-B.D. thanks Eliot Quataert and J.E.G. Peek for helpful and insightful discussions. Support for this work was provided by NASA through grant SAO 08700135 issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory on behalf of NASA under contract NAS8-03060, and through grant HST-GO-11130.01-A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555.
Publisher Copyright:
© 2009 The American Astronomical Society. All rights reserved. Printed in the U.S.A.
PY - 2009/6/20
Y1 - 2009/6/20
N2 - 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.
AB - 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.
KW - Galaxies: Seyfert
KW - Galaxies: active
KW - Galaxies: nuclei
KW - Galaxies: statistics
KW - X-rays: galaxies Online-only material: color figures
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UR - http://www.scopus.com/inward/citedby.url?scp=70450122973&partnerID=8YFLogxK
U2 - 10.1088/0004-637X/698/2/1515
DO - 10.1088/0004-637X/698/2/1515
M3 - Article
AN - SCOPUS:70450122973
SN - 0004-637X
VL - 698
SP - 1515
EP - 1522
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 2
ER -