The Hard X-Ray Spectrum as a Probe for Black Hole Growth in Radio-Quiet Active Galactic Nuclei

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/L_Edd) 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 L_X can provide an estimate of L/L_Edd and black hole mass (M_BH) 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/L_Edd 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 (α_ox) depends primarily on optical-UV luminosity rather than on L/L_Edd in a sample of RQ AGNs spanning 5 orders of magnitude in luminosity and over 2 orders of magnitude in L/L_Edd. 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^{+ 0.6}_−0.5 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.