The X-ray–UV luminosity relation of eROSITA quasars

¹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
² INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, Via A.Corti 12, 20133 Milano, Italy
³ Dipartimento di Fisica e Astronomia, Universita‘ di Firenze, Via G. Sansone 1, I-50019 Sesto Fiorentino, Firenze, Italy
⁴ INAF “Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125 Firenze, Italy
⁵ European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
⁶ Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy

^⋆ Corresponding author: andrea.sacchi@inaf.it

Received: 1 August 2025
Accepted: 15 October 2025

Abstract

The nonlinear relation between the UV and X-ray luminosity in quasars has been studied for decades. However, as no comprehensive model can yet 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 the eROSITA all-sky survey (eRASS1). We focus on the subset of quasars bright enough in the optical/UV band to avoid an “Eddington bias” toward X-ray brighter-than-average spectral energy distributions. The final samples include 1248 and 519 sources for eFEDS and eRASS1, up to redshift z ≈ 3 and z ≈ 1.5, respectively. We find that the X-ray–UV luminosity relation does not evolve significantly 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, due to the limited redshift range and statistics of the current samples, we cannot obtain significant cosmological constraints yet. We show how this is going to change with the future releases of eROSITA data.