Quasars as high-redshift standard candles

Understand

The non-linear relation between the X-ray and ultraviolet (UV) luminosity in quasars has been used to derive quasar distances and to build a Hubble diagram at redshifts up to $z\sim$ 7.

• This cosmological application is based on the assumption of independence of the relation on redshift and luminosity.
• We want to test the reliability of this hypothesis by studying the spectroscopic properties of high-redshift quasars in the X-ray and UV bands.
• We performed a one-by-one analysis of a sample of 130 quasars at $z>$ 2.5 with high-quality X-ray and UV spectroscopic observations.