Quasars as high redshift standard candles: the LX-LUV relation at high redshift

A tight non-linear relation between the X-ray and the optical-ultraviolet luminosity has been observed in AGN over 5 orders of magnitude and up to high redshift. This suggests a coupling between the disk, emitting the primary radiation in the UV band, and the hot corona emitting in the X-ray. In this work, we have studied the Lx-Luv relation for a sample of high-redshift (z>4) quasars, selected on the basis of their spectral properties and the quality of the available observations in both X-ray and optical/UV bands. The relation shows no-evidence of evolution with redshift, which indicates a universal physical mechanism regulating the energy transfer process in the inner regions, and the observed dispersion is lower than in previous estimates. 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 that can provide an important contribution in the determination of the cosmological parameters, probing cosmological time not achievable with other observational methods.