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

TL;DR

This paper explores the use of the X-ray photon index as a tool to estimate the bolometric correction in AGNs, enabling potential applications in understanding black hole properties and cosmology, with initial promising results but notable uncertainties.

Contribution

It introduces a correlation between the X-ray photon index and bolometric correction in AGNs, providing a new method to estimate AGN luminosities and black hole masses across different redshifts.

Findings

Correlation between  and bolometric correction established.

Estimated luminosities agree with observed values for some AGNs.

Uncertainties remain significant, requiring further research.

Abstract

We propose the rest-frame 2-10 keV photon index, \ga, acting as an indicator of the bolometric correction, \lb/$L_{\rm 2-10keV}$ (where \lb~ is the bolometric luminosity and $L_{\rm 2-10keV}$ is the rest-frame 2-10 keV luminosity), in radio-quiet active galactic nuclei (AGNs). Correlations between \ga~ 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 $L_{\rm 2-10keV}$, 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, \ga~ for high-redshift AGNs can be used to determine their true \lb~ and $L_{\rm 2-10keV}$, 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 $L_{\rm 2-10keV}$ estimated from \ga~ for the brightest Type I AGNs with $z<1$ in the Lockman Hole is generally in agreement with the observed $L_{\rm 2-10keV}$. However, there are still many uncertainties, such as the accurate determination of the intrinsic \ga~ 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.