Hard X-ray photon index as an indicator of bolometric correction in active galactic nuclei

Abstract

We propose the rest-frame 2-10 keV photon index, Γ 2-10 keV, acting as an indicator of the bolometric correction, L bol/L2-10 keV (where Lbol is the bolometric luminosity and L2-10 keV is the rest-frame 2-10 keV luminosity), in radio-quiet active galactic nuclei (AGNs). Correlations between Γ2-10 keV and both bolometric correction and Eddington ratio are presented, based on simultaneous X-ray, UV, and optical observations of reverberation-mapped AGNs. These correlations can be compared with those for high-redshift AGNs to check for any evolutionary effect. Assuming no evolutionary effect in AGNs' spectral properties, together with the independent estimates of L2-10 keV, the bolometric correction, Eddington ratio, and black hole (BH)mass can all be estimated from these correlations for high-redshift AGNs, with the mean uncertainty of a factor of 2-3. If there are independent estimates of BH masses, Γ2-10 keV for high-redshift AGNs can be used to determine their true Lbol and L2-10 keV, and in conjunction with the redshift can be potentially used to place constraints on cosmology by comparison with the rest-frame 2-10 keV flux. We find that the true L2-10 keV estimated from Γ2-10 keV for the brightest Type I AGNs with z < 1 in the Lockman Hole is generally in agreement with the observed L 2-10 keV. However, there are still many uncertainties, such as the accurate determination of the intrinsic Γ2-10 keV for distant AGNs and the large uncertainty in the luminosities obtained, which call for significant further study before "AGN cosmology" can be considered a viable technique. © 2010. The American Astronomical Society. All rights reserved.