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

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 <= z <= 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 standardize these QSOs depend on the cosmological model used to determine these parameters, i.e. it appears that the full QSO data set includes 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 less than or similar to 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.

Automated summary

The study found that the parameters of the X-ray and UV luminosities relation for the full QSO data set depend on the cosmological model used. Subset of QSO data with lower redshifts show a relationship between X-ray and UV flux independent of cosmological model, which can be used to constrain cosmological parameters. The cosmological constraints from these lower redshift QSO data subsets are mostly consistent with other measurements but weaker in strength.