The X-ray to optical-UV luminosity ratio of X-ray selected Type 1 AGN in XMM-COSMOS

TL;DR

This study analyzes the X-ray to optical-UV luminosity ratio of 545 X-ray selected Type 1 AGN from XMM-COSMOS, revealing correlations with redshift, luminosity, and Eddington ratio, and providing a method to estimate bolometric corrections.

Contribution

It presents the first comprehensive analysis of the X-ray to optical-UV luminosity ratio in a large, diverse sample of X-ray selected Type 1 AGN, including new correlations with Eddington ratio.

Findings

Significant correlation between αox and optical luminosity.

Bolometric correction increases with Eddington ratio.

αox correlates with Eddington ratio, rising at higher ratios.

Abstract

We present a study of the X-ray to optical properties of a sample of 545 X-ray selected Type 1 AGN, from the XMM-COSMOS survey, over a wide range of redshifts ($0.04<\z<4.25$) and X-ray luminosities ($40.6 \leq \Log \Lhard \leq 45.3$). About 60% of them are spectroscopically identified Type 1 AGN, while the others have a reliable photometric redshift and are classified as Type 1 AGN on the basis of their multi-band Spectral Energy Distributions. We discuss the relationship between UV and X-ray luminosity, as parameterized by the $\alphaox$ spectral slope, and its dependence on redshift and luminosity. We compare our findings with previous investigations of optically selected broad-line AGN (mostly from SDSS). A highly significant correlation between $\alphaox$ and $\lo$ is found, in agreement with previous investigations of optically selected samples. We calculate bolometric corrections, $\kbol$, for the whole sample using hard X-ray luminosities ($\Lhard$), and the Eddington ratios for a subsample of 150 objects for which black hole mass estimates are available. We confirm the trend of increasing bolometric correction with increasing Eddington ratio as proposed in previous works. A tight correlation is found between $\alphaox$ and $\kbol$, which can be used to estimate accurate bolometric corrections using only optical and X-ray data. We find a significant correlation between $\alphaox$ and Eddington ratio, in which $\alphaox$ increases for increasing Eddington ratios.