Speaker
Description
Unobscured, radio-quiet quasars define a tight correlation between X-ray and UV luminosities (the Lx-Luv relation), widely interpreted as the signature of disc-corona coupling. However, the physical origin of the scatter around the relation remains unclear, with X-ray-weak quasars representing its most extreme deviations. We assemble a statistically robust sample of more than 5000 quasars with homogeneous SDSS UV/optical and Chandra X-ray spectroscopy, and use it to explore how the properties of the central engine and its ionising output, as traced by high-ionisation gas, vary across the Lx-Luv plane. We focus on the C IV, He II, and [O III] emission lines, which trace gas on parsec and kiloparsec scales, and analyse how their luminosities, equivalent widths, and blueshifts vary as a function of LX, LUV, αox, and Δαox, the offset from the relation. We find that X-ray-weak quasars are, on average, over-luminous in high-ionisation lines at fixed LX, and that line strength and kinematics vary systematically with Δαox , most clearly for C IV and [O III]. These trends suggest that the offset from the Lx-Luv relation traces not only the disc-corona balance, but also the ionising continuum seen by gas on both BLR and NLR scales. We also present a complementary machine-learning analysis aimed at identifying which UV spectral properties, particularly those of C IV, are most predictive of the offset from the relation in a multivariate framework.