Determining the range of validity of quasar X-ray and UV flux measurements for constraining cosmological model parameters

TL;DR

This study assesses the validity of using quasar X-ray and UV flux measurements to constrain cosmological models, finding that only low-redshift subsets reliably standardize for cosmological inference.

Contribution

It demonstrates that the $L_X-L_{UV}$ relation depends on the cosmological model at high redshifts, identifying a reliable low-redshift subset for cosmological constraints.

Findings

Full QSO data set includes non-standardizable quasars at high redshift.

Low-redshift QSO subsets obey the $L_X-L_{UV}$ relation independently of cosmology.

Cosmological constraints from low-redshift QSOs are consistent with other methods but weaker.

Abstract

We use six different cosmological models to study the recently-released compilation of X-ray and UV flux measurements of 2038 quasars (QSOs) which span the redshift range $0.009 \leq z \leq 7.5413$. We find, for the full QSO data set, that the parameters of the X-ray and UV luminosities $L_X-L_{UV}$ relation used to standardized these QSOs depend on the cosmological model used to determine these parameters, i.e, it appears that the full QSO data set include QSOs that are not standardized and so cannot be used for the purpose of constraining cosmological parameters. Subsets of the QSO data, restricted to redshifts $z \lesssim 1.5-1.7$ obey the $L_X-L_{UV}$ relation in a cosmological-model-independent manner, and so can be used to constrain cosmological parameters. The cosmological constraints from these lower-$z$, smaller QSO data subsets are mostly consistent with, but significantly weaker than, those that follow from baryon acoustic oscillation and Hubble parameter measurements.