The X-Ray Energy Dependence of the Relation Between Optical and X-Ray Emission in Quasars

Abstract

We develop a new approach to the well-studied anti-correlation between the optical-to-X-ray spectral index, α ox , and the monochromatic optical luminosity, l opt . By cross-correlating the SDSS DR5 quasar catalog with the XMM-Newton archive, we create a sample of 327 quasars with X-ray S/N > 6, where both optical and X-ray spectra are available. This allows α ox to be defined at arbitrary frequencies, rather than the standard 2500Å and 2 keV. We find that while the choice of optical wavelength does not strongly influence the α ox −l opt relation, the slope of the relation does depend on the choice of X-ray energy. The slope of the relation becomes steeper when α ox is defined at low (∼ 1 keV) X-ray energies. This change is significant when compared to the slope predicted by a decrease in the baseline over which α ox is defined. The slopes are also marginally flatter than predicted at high (∼ 10 keV) X-ray energies. Partial correlation tests show that while the primary driver of α ox is l opt , the Eddington ratio correlates strongly with α ox when l opt is taken into account, so accretion rate may help explain these results. We combine the α ox − l opt and Γ-L bol /L Edd relations to naturally explain two results: 1) the existence of the Γ − l x relation as reported in Young et al. (2009) and 2) the lack of a Γ − l opt relation. The consistency of the optical/X-ray correlations establishes a more complete framework for understanding the relation between quasar emission mechanisms. We also discuss two correlations with the hard X-ray bolometric correction, which we show correlates with both α ox and Eddington ratio. This confirms that an increase in accretion rate correlates with a decrease in the fraction of up-scattered disk photons.

Variability and the X-ray/UV ratio of active galactic nuclei

“…Indeed, it is lower than those found by Strateva et al (2005, e.g.) (0.14) and by Young et al (2010) (0.16), but slightly higher than that evaluated by Gibson et al (2008) (0.10). Values found in previous simultaneous studies are also of the same order, e.g.…”

“…For example, both Lusso et al (2010) and G10 found evidence of a decrease of α ox with Eddington ratio. Moreover, Young et al (2010) found a significant partial anti-correlation with the Eddington ratio when the dependence on L UV is accounted for; the significance increases if the X-ray energy in the α ox definition is increased.…”

“…Both populations are therefore usually removed from the analysis of the α ox − L UV anti-correlation (e.g. Just et al 2007;Gibson et al 2008;Young et al 2010;Vagnetti et al 2010).…”

Quasars as standard candles

“…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 .…”

Black-hole masses of type 1 AGN in theXMM-Newtonbright serendipitous survey

“…see Young et al 2010;Vasudevan & Fabian 2009;Grupe et al 2010;Lusso et al 2012, and references therein). To this end, statistically complete and well-defined samples of AGNs equipped with X-ray spectral data and with a reliable estimate of the accretion parameters (SMBH mass, the absolute accretion rate, the accretion rate normalized to the Eddington limit) are required.…”