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, Γ2–10 keV, acting as an indicator of the bolometric correction, Lbol/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 L2–10 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.