Speaker
Description
The non-linear relation between X-ray and UV luminosities in quasars, usually parametrised as log(LX) = γ log(LUV) + β, is one of the most direct observational signatures of the coupling between the accretion disc and the X-ray corona. Beyond its astrophysical relevance, its non-linearity makes it a powerful tool for measuring cosmological distances, allowing quasars to be used as standardisable candles up to redshifts not reached by other probes. In this talk, I will present results from a new sample of ~2500 quasars with X-ray and UV measurements extending up to z~5. I will discuss how we carefully build the sample to avoid biases from reddening, obscuration, and the Eddington bias, and show that the relation parameters do not evolve with redshift, confirming that the disc-corona coupling in quasars is remarkably stable across redshifts, black hole masses, and luminosities. Furthermore, I will show how the dispersion for this new sample reaches a new record low of ~0.15 dex on average, and discuss how that can be almost entirely accounted for by variability and inclination effects, leaving little room for any intrinsic scatter. Finally, I will discuss how this new, improved sample can strengthen the cosmological implementation of quasars in combination with other probes (supernovae, CMB), and how to ensure such analyses remain free from systematic biases.