TY - JOUR
T1 - The hard X-ray spectrum as a probe for black hole growth in radio-quiet active galactic nuclei
AU - Shemmer, Ohad
AU - Brandt, W. N.
AU - Netzer, Hagai
AU - Maiolino, Roberto
AU - Kaspi, Shai
PY - 2008/7/20
Y1 - 2008/7/20
N2 - 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.
AB - 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.
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U2 - 10.1086/588776
DO - 10.1086/588776
M3 - Article
AN - SCOPUS:48749096674
SN - 0004-637X
VL - 682
SP - 81
EP - 93
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 1
ER -