Quasars as standard candles

Abstract

Context. A tight non-linear relation exists between the X-ray and UV emission in quasars (i.e. L X ∝ L γ UV ), with a dispersion of ∼0.2 dex over approximately three orders of magnitude in luminosity. Such observational evidence has two relevant consequences:(1) an ubiquitous physical mechanism must regulate the energy transfer from the accretion disc to the X-ray emitting corona, and (2) the non-linearity of the relation provides a new, powerful way to estimate the absolute luminosity, turning quasars into a new class of standard candles. Aims. Here we propose a modified version of this relation, which involves the emission line full-width half maximum, L X ∝ Lˆγ UV υˆβ fwhm . Methods. We interpret this new relation through a simple, ad-hoc model of accretion disc corona, derived from previous works in the literature where it is assumed that reconnection and magnetic loops above the accretion disc can account for the production of the primary X-ray radiation. Results. We find that the monochromatic optical-UV (2500 Å) and X-ray (2 keV) luminosities depend on the black hole mass and accretion rate as L UV ∝ M 4/3 BH (Ṁ/Ṁ Edd ) 2/3 and L X ∝ M 19/21 BH (Ṁ/Ṁ Edd ) 5/21 , respectively. Assuming a broad line region size function of the disc luminosity R blr ∝ L 0.5 disc we finally have that L X ∝ L 4/7 UV υ 4/7 fwhm . Such relation is remarkably consistent with the slopes and the normalisation obtained from a fit of a sample of 545 optically selected quasars from SDSS DR7 cross matched with the latest XMM-Newton catalogue 3XMM-DR6. Conclusions. The homogeneous sample used here has a dispersion of 0.21 dex, which is much lower than previous works in the literature and suggests a tight physical relation between the accretion disc and the X-ray emitting corona. We also obtained a possible physical interpretation of the L X − L UV relation (considering also the effect of υ fwhm ), which puts the determination of distances based on this relation on a sounder physical grounds. The proposed new relation does not evolve with time, and thus it can be employed as a cosmological indicator to robustly estimate cosmological parameters.

NuSTAR spectral analysis of two bright Seyfert 1 galaxies: MCG +8-11-11 and NGC 6814

“…These values are consistent, with the values of α ox we found before. Moreover the sources follow also the trend of the L X −L UV of Lusso & Risaliti (2017).…”

The correlations between optical/UV broad lines and X-ray emission for a large sample of quasars

“…Figure 2 illustrates that our Full quasar sample spans the luminosity range representative of all SDSS quasars, and thus our sample also allows investigation of the relationship between X-ray strength and luminosity (e.g. Steffen et al 2006;Just et al 2007;Lusso & Risaliti 2017) over a wide luminosity range. The WLQs tend to have brighter absolute magnitudes; however, this trait is a selection effect since bright WLQs were selected for X-ray observation to allow for economical snapshot observations with Chandra.…”

“…The nature of this corona is still uncertain; however, correlations between the optical/UV and X-ray emission in quasars have been used as a diagnostic to compare the coronal region and the accretion disk (e.g. Steffen et al 2006;Just et al 2007;Lusso & Risaliti 2017). Furthermore, the ionizing X-ray radiation has been linked to the emission-line properties in rest-frame optical/UV spectra (e.g.…”

The final SDSS-IV/SPIDERS X-ray point source spectroscopic catalogue

“…Recently, Risaliti & Lusso (2015); Lusso & Risaliti (2017) proposed a method to construct quasar standard candles. It relies on the fact that exist an intrinsic non-linear relation between the UV emission from the accretion disk and the X-ray emission from the surrounding corona of the AGN.…”

“…Our current understanding of the disc-coronae and its non-linear scaling between UV and X-ray luminosity is not yet sufficient to prove this method in details. In the literature, there is skepticism about the physical disc-coronae model from Lusso & Risaliti (2017) to account for this relation. For example, Kubota & Done (2018); Panda et al (2019) propose a model that is in agreement with the LX-LUV relation from Lusso.…”