Abstract
We propose a modified version of the observed non-linear relation between the X-ray (2 keV) and the ultraviolet (2,500 A) emission in quasars (i.e., LX∝LγUV) which involves the full width at half-maximum, FWHM, of the broad emission line, i.e., LX∝LγˆUV FWHMβˆ. By analyzing a sample of 550 optically selected non-jetted quasars in the redshift range of 0.36–2.23 from the Sloan Digital Sky Survey cross matched with the XMM-Newton catalog 3XMM-DR6, we found that the additional dependence of the observed LX − LUV correlation on the FWHM of the MgII broad emission line is statistically significant. Our statistical analysis leads to a much tighter relation with respect to the one neglecting FWHM, and it does not evolve with redshift. We interpret this new relation within an accretion disc corona scenario where reconnection and magnetic loops above the accretion disc can account for the production of the primary X-ray radiation. For a broad line region size depending on the disc luminosity as Rblr∝L0.5disc, we find that LX∝L4/7UV FWHM4/7, which is in very good agreement with the observed correlation.
Highlights
One of the observational evidences for the link between the accretion disc and the X-ray corona in active galactic nuclei (AGN) is given by the observed non-linear correlation between the monochromatic ultraviolet luminosity at 2,500 Å (LUV) and the one in the X–rays at 2 keV (LX)
Our modified LX − LUV relationship in quasars, which takes into account the full-width half-maximum of the quasar emission line radiation pressure in the disc
Such a relation is remarkably consistent with the fit obtained from a sample of 550 optically selected quasars from SDSS DR7 cross matched with the XMM–Newton catalog 3XMM-DR6
Summary
One of the observational evidences for the link between the accretion disc and the X-ray corona in active galactic nuclei (AGN) is given by the observed non-linear correlation between the monochromatic ultraviolet luminosity at 2,500 Å (LUV) and the one in the X–rays at 2 keV (LX). Some attempts at explaining the quasar X-ray spectra were based upon the reprocessing of radiation from a non-thermal electron-positron pair cascade (e.g., Svensson, 1982, 1984; Zdziarski et al, 1990) Another possibility rests on a two-phase accretion disc model, where a fraction f of gravitational power is dissipated via buoyancy and reconnection of magnetic fields in a uniform, The Physical Relation between Disc and Coronal Emission hot (Tcor ∼ 100 keV ∼ 109 K) plasma close to the cold opaque disc (Haardt and Maraschi, 1991, 1993; Svensson and Zdziarski, 1994; Di Matteo, 1998; Rózanska and Czerny, 2000). Both γz and βz slopes do not show any significant evolution with time and they are consistent with a constant value in the redshift interval 0.3–2
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