Abstract

Exploring new/alternative standard candles is crucial for advancing cosmology. One spanning a wide redshift range, like quasars (QSOs), can provide orthogonal constraints on model parameters, among which H0, with its current tension, is particularly relevant. The 2 keV to 2500 Å flux ratio in QSOs shows great potential as a cosmological tool (e.g., Lusso+20, Sacchi+22) if suitable subsets are identified, akin to the corrections applied to supernovae. We investigate various properties of QSOs to deepen our understanding of QSO physics and refine the selection criteria for a new cosmological quasar sample. We specifically examine the most luminous QSOs exhibiting strong outflows in the C IV emission line to clarify the relation between the strength of the outflows and X-ray emission, thereby enhancing our understanding of the mechanisms within active galactic nuclei (AGN) engines. Using targeted observations with Chandra, we determine the slope of the 2 keV to 2500 Å luminosity relation and explore how the C IV blueshift correlates with the steepness of the UV-to-X-ray spectral energy distribution. To gain insights into the fundamental processes underlying the 2 keV to 2500 Å luminosity relation, we analyse the slopes of the relation measured using multi-epoch XMM observations of specific sources. Finally, to investigate the correlations between the dispersion of the 2 keV to 2500 Å luminosity relation, fundamental AGN properties, and optical variability, we assess several variability features from the ZTF optical light curves and conduct a dimension reduction analysis.