Quasar UV/X-ray relation luminosity distances are shorter than reverberation-measured radius-luminosity relation luminosity distances

Abstract

We use measurements of 59/58 quasars (QSOs), over a redshift range 0.0041 ≤ z ≤ 1.686, to do a comparative study of the radius-luminosity (R - L) and X-ray-UV luminosity (L_X - L_UV) relations and the implication of these relations for cosmological parameter estimation. By simultaneously determining R - L or L_X - L_UV relation parameters and cosmological parameters in six different cosmological models, we find that both R - L and L_X - L_UV relations are standardizable but provide only weak cosmological parameter constraints, with L_X - L_UV relation data favouring larger current non-relativistic matter density parameter Ω_m0 values than R - L relation data and most other available data. We derive L_X - L_UV and R - L luminosity distances for each of the sources in the six cosmological models and find that L_X - L_UV relation luminosity distances are shorter than R - L relation luminosity distances as well as standard flat ΛCDM model luminosity distances. This explains why L_X - L_UV relation QSO data favour larger Ω_m0 values than do R - L relation QSO data or most other cosmological measurements. While our sample size is small and only spans a small z range, these results indicate that more work is needed to determine whether the L_X - L_UV relation can be used as a cosmological probe.