The Tight Relation Between X-Ray and Ultraviolet Luminosity of Quasars

Abstract

The observed relation between the soft X-ray and the optical-ultraviolet emission in active galactic nuclei (AGNs) is nonlinear and it is usually parametrized as a dependence between the logarithm of the monochromatic luminosity at 2500 Å 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 four 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.

Quasars as standard candles

“…Here we present a modified version of the L X −L UV relation which involves the emission line full-width half maximum (υ fwhm ), L X ∝ Lγ UV υβ fwhm . The homogeneous sample used here has a dispersion of δ ∼ 0.21 and, based on the analysis presented in Lusso & Risaliti (2016), we expect that this dispersion can be further reduced if precise, well calibrated X-ray observations are performed. In LR16 we estimated an intrinsic dispersion not higher than δ 0.18 − 0.19, consistent with the one estimated with the best quasar sample with multiple observations only.…”

“…The non-linear relationship between L X and L UV (parameterised as log L X = γ log L UV + β) has been found in both optically and X-ray selected AGN samples and exhibits a slope γ around 0.5 − 0.7 (Vignali et al 2003;Strateva et al 2005;Steffen et al 2006;Just et al 2007;Green et al 2009;Lusso et al 2010;Young et al 2010;Marchese et al 2012;Jin et al 2012), implying that optically bright AGN emit relatively less X-rays than optically faint AGN. Such relation is independent of redshift, it is very tight (∼ 0.2 dex observed dispersion, Lusso & Risaliti 2016), and it has also been employed as a distance indicator to estimate cosmological parameters such as Ω M and Ω Λ . Thanks to the L X − L UV relationship (or, more precisely, its version with fluxes), Risaliti & Lusso (2015) have built the first quasar Hubble diagram which extends up to z ∼ 6, in excellent agreement with the analogous Hubble diagram for Type Ia supernovae in the common redshift range (i.e.…”

Universal bolometric corrections for active galactic nuclei over seven luminosity decades

“…Black hexagons are the average bolometric correction values, in bins of bolometric luminosity. The black solid line is our best fit solution; the brown solid line is the analytical prediction obtained by assuming the relation between the X-ray luminosity and the optical luminosity by Lusso & Risaliti (2016) our best-fit relation (Equation 2). The normalized values (black hexagons) and the best-fit (dashed line) are shown in Figure 8 (top panel), and suggest no trend with redshift.…”

“…Some authors have investigated the evolution of the α OX spectral index (see e.g., Kelly et al 2008;Vasudevan et al 2009;Lusso et al 2010;Marchese et al 2012;Lusso & Risaliti 2017), which should provide a hint about the nature of the energy generation mechanism in AGN. Many of them report that the UV-to-X-ray SED shows no significant dependence on redshift, while the primary dependence is on the UV luminosity (Vignali et al 2003;Steffen et al 2006;Lusso & Risaliti 2016). This supports a scenario in which the mechanism responsible for the nuclear emission of the AGN remains identical at any redshift, while it correlates with the AGN luminosity: in particular, the ratio between the X-ray luminosity and the UV/optical one decreases with increasing luminosity (see e.g.…”

The WISSH quasars project

“…In addition, many studies report that the UV-to-X-ray spectral energy distribution (typically described by the relationship between the 2500 Å and 2 keV monochromatic luminosities, i.e. α OX ) depends primarily upon the UV luminosity (Avni & Tananbaum 1982;Vignali et al 2003;Steffen et al 2006;Lusso et al 2010;Lusso & Risaliti 2016), while the redshift dependence is weak. These results lend support to a scenario whereby the mechanism responsible for the primary continuum emission in AGN remains almost identical from z ≈ 0 to z ≈ 6, while the more the AGN luminosity increases, the less the energy in the X-ray band is emitted relative to that in the UV/optical range.…”