The hard X-ray spectrum as a probe for black hole growth in radio-quiet active galactic nuclei

Ohad Shemmer, W. N. Brandt, Hagai Netzer, Roberto Maiolino, Shai Kaspi

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201 Scopus citations


We study the hard X-ray spectral properties of 10 highly luminous radio-quiet (RQ) active galactic nuclei (AGNs) at z = 1:3 3:2, including new XMM-Newton observations of four of these sources. We find a significant correlation between the normalized accretion rate (L/LEdd) and the hard X-ray photon index (Γ) for 35 moderate- to high-luminosity RQ AGNs, including our 10 highly luminous sources. Within the limits of our sample, we show that a measurement of Γ and LX can provide an estimate of L/LEdd and black hole mass (MBH) with a mean uncertainty of a factor of ≲3 on the predicted values of these properties. This may provide a useful probe for tracing the history of BH growth in the universe, utilizing samples of X-ray-selected AGNs for which L/LEdd and M BH have not yet been determined systematically. It may prove to be a useful way to probe BH growth in distant Compton-thin type 2 AGNs. We also find that the optical-X-ray spectral slope (αox) depends primarily on optical-UV luminosity rather than on L/LEdd in a sample of RQ AGNs spanning 5 orders of magnitude in luminosity and over 2 orders of magnitude in L/LEdd. We detect a significant Compton-reflection continuum in two of our highly luminous sources, and in the stacked X-ray spectrum of seven other sources with similar luminosities, we obtain a mean relative Compton reflection of R = 0:9-0.5+0.6 and an upper limit on the rest-frame equivalent width of a neutral Fe Kα line of 105 eV. We do not detect a significant steepening of the X-ray power-law spectrum below rest-frame 2 keV in any of our highly luminous sources, suggesting that a soft-excess feature, commonly observed in local AGNs, either does not depend strongly on L/L Edd, or is not accessible at high redshifts using current X-ray detectors.

Original languageEnglish (US)
Pages (from-to)81-93
Number of pages13
JournalAstrophysical Journal
Issue number1
StatePublished - Jul 20 2008

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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