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 (LX − LUV) relations and the implication of these relations for cosmological parameter estimation. By simultaneously determining R − L or LX − LUV relation parameters and cosmological parameters in six different cosmological models, we find that both R − L and LX − LUV relations are standardizable but provide only weak cosmological parameter constraints, with LX − LUV relation data favoring larger current non-relativistic matter density parameter Ωm0 values than R − L relation data and most other available data. We derive LX − LUV and R − L luminosity distances for each of the sources in the six cosmological models and find that LX − LUV relation luminosity distances are shorter than R − L relation luminosity distances as well as standard flat ΛCDM model luminosity distances. This explains why LX − LUV relation QSO data favor larger Ωm0 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 LX − LUV relation can be used as a cosmological probe.

Context. We present the results of the reverberation monitoring of the MgII broad line and FeII pseudocontinuum for the luminous quasar CTS C30.10 (z = 0.90052) with the Southern African Large Telescope in 2012-2021. Aims. We aimed at disentangling the MgII and UV FeII variability and the first measurement of UV FeII time delay for a distant quasar. Methods. We used several methods for the time-delay measurements and determined the FeII and MgII time delays. We also performed a wavelength-resolved time delay study for a combination of MgII and FeII in the 2700 -2900 Å rest-frame wavelength range. Results. We obtain a time delay for MgII of 275.5 +12.4 −19.5 days in the rest frame, and we have two possible solutions of 270.0 +13.8 −25.3 days and 180.3 +26.6 −30.0 in the rest frame for FeII. Combining this result with the old measurement of FeII UV time delay for NGC 5548, we discuss for first time the radius-luminosity relation for UV FeII with the slope consistent with 0.5 within the uncertainties. Conclusions. Because the FeII time delay has a shorter time-delay component but the lines are narrower than MgII, we propose that the line-delay measurement is biased toward the part of the broad line region (BLR) facing the observer. The bulk of the Fe II emission may arise from the more distant BLR region, however, the region that is shielded from the observer.

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