The relation between X-ray and UV emission in quasars

Abstract

The correlation between the X-ray and UV luminosities observed in quasars, spanning a wide redshift range and holding true for several decades in both spectral bands, suggests the presence of a universal mechanism governing the transfer of energy from the accretion disc to the hot corona. In this study, we leverage X-ray spectroscopic data extracted from the Chandra Source Catalog 2.0 for a sample of over $2000$ quasars from the Sloan Digital Sky Survey Data Release 14 (SDSS DR14). Our analysis reveals a reduced intrinsic dispersion in the $L_{\rm{X}}-L_{\rm{UV}}$ relation at higher redshifts ($\delta < 0.2$ dex) compared to previous studies relying on photometric data from catalogs. Additionally, our findings confirm the stability of this relation up to redshifts of approximately $4.5$. The $L_{\rm{X}}-L_{\rm{UV}}$ relation can also serve as a tool to investigate the physics of accretion by identifying outliers - sources that exhibit a different state of the accretion disc-hot corona system compared to the average population. For instance, X-ray-weak quasars are sources with reduced X-ray emissions due to a radiatively inefficient state of the corona, and their optical properties suggest the presence of a powerful accretion disc wind. The wealth of spectroscopic data available in the CSC 2.0-SDSS catalogs opens up the opportunity for a more comprehensive exploration of the central engine in AGN.