The tight relation between X-ray and ultraviolet luminosity of quasars

TL;DR

This study reveals a very tight, intrinsic correlation between X-ray and ultraviolet luminosity in quasars, suggesting a fundamental physical link between the accretion disk and corona across a wide luminosity range.

Contribution

The paper demonstrates that selecting homogeneous, unobscured quasars reduces the observed dispersion, indicating a more intrinsic and universal X-ray/UV luminosity relation.

Findings

Dispersion drops to ~0.21-0.24 dex with careful sample selection.

Residual dispersion attributed to variability and measurement errors.

The intrinsic dispersion is likely less than 0.21 dex.

Abstract

The observed relation between the soft X-ray and the optical-ultraviolet emission in active galactic nuclei (AGN) is non-linear and it is usually parametrized as a dependence between the logarithm of the monochromatic luminosity at 2500 {\AA} and at 2 keV. Previous investigations have found that the dispersion of this relation is rather high (~0.35-0.4 in log units), which may be caused by measurement uncertainties, variability, and intrinsic dispersion due to differences in the AGN physical properties (e.g. different accretion modes). We show that, once optically-selected quasars with homogeneous SED and X-ray detection are selected, and dust reddened and/or gas obscured objects are not included, the measured dispersion drops to significantly lower values (i.e. ~0.21-0.24 dex). We show that the residual dispersion is due to some extent to variability, and to remaining measurement uncertainties. Therefore, the real physical intrinsic dispersion should be <0.21 dex. Such a tight relation, valid over 4 decades in luminosity, must be the manifestation of an intrinsic (and universal) physical relation between the disk, emitting the primary radiation, and the hot electron corona emitting X-rays.