The Relationship Between the High Energy Continuum and Emission Lines in QSOs: A Low-Redshift Sample

Abstract

Quasar emission lines are sensitive to both the luminosity and shape of the quasars' high energy continuum - primarily in the soft X-ray and the (obscured) extreme ultraviolet. We initiate the first large-scale, multi-line investigation of correlations between the QSO soft X-ray continuum and UV line emission, using an IUE/Einstein sample. We characterize the distributions of line equivalent and velocity widths (EW and FWHM) of 85 QSOs in the IUE atlas of Lanzetta et al. (1993), using a new error analysis for objective, automated line measurements, and contrast measured line parameters to the QSO continuum spectral energy distributions (SEDs). We find significant correlations between EW and UV luminosity (e.g., the well-studied Baldwin effect) for Lya, CIV, HeII, and CIII. EW(CIII) and EW(HeII) show previously unreported correlations with Lx which for CIII appears to be primary. The line ratios CIII/Lya and HeII/Lya both show strongest dependence on Lx. EW(Lya) correlates strongly with spectral slopes α_{uv} and α_{ox}, but NOT with Lx. We argue that one simple geometrical interpretation of the Baldwin effect (BEff) as a result of a distribution of disk inclinations is not plausible. The BEff weakens or disappears when line emission is correctly compared to the luminosity in the continuum bandpass relevant to its production, which suggests that the BEff is due to a change in SED with luminosity. Our prediction that no BEff relative to X-ray luminosity should be found for FeII or MgII emission lines, will be tested in a later paper, extended to a wider redshift/luminosity range.