The X-ray$-$UV Luminosity Relation of eROSITA Quasars

TL;DR

This study analyzes the non-linear X-ray and UV luminosity relation in quasars using eROSITA data, finding no significant evolution with redshift and highlighting its potential for cosmological applications.

Contribution

It provides a comprehensive analysis of the X-ray–UV luminosity relation in quasars with eROSITA data, confirming its stability and potential for cosmology.

Findings

No significant redshift evolution of the relation

Intrinsic dispersion of about 0.2 dex

Potential for cosmological applications with future data

Abstract

The non-linear relation between the UV and X-ray luminosity in quasars has been studied for decades. However, as we lack a comprehensive model able to explain it, its investigation still relies on observational efforts. This work focuses on optically selected quasars detected by eROSITA. We present the properties of the sources collected in the eROSITA early data release (eFEDS) and those resulting from the first six months of eROSITA all-sky survey (eRASS1). We focus on the subset of quasars bright enough in the optical/UV band to avoid an ''Eddington bias'' towards X-ray brighter-than-average spectral energy distributions. The final samples include 1,248 and 519 sources for eFEDS and eRASS1, up to redshift $z\approx3$ and $z\approx1.5$, respectively. We found that the X-ray$-$UV luminosity relation shows no significant evolution with redshift, and its slope is in perfect agreement with previous compilations of quasar samples. The intrinsic dispersion of the relation is about 0.2 dex, which is small enough for possible cosmological applications. However, the limited redshift range and statistics of the current samples do not allow us to obtain significant cosmological constraints yet. We show how this is going to change with the future releases of the eROSITA data.