Quasar discs - III. Line and continuum correlations

We analyse a very large sample of Active Galactic Nuclei (AGNs) and discuss several of the observed line and continuum correlations. We show that the luminosity of the Lyα and C IV λ1549 lines do not increase as fast as the continuum luminosity, and that the slope of the Baldwin relationship is a function of the luminosity range of the sample under study. Thin AGN accretion discs can provide one explanation for those correlations, as we show by computing the continuum emitted by such systems and combining it with photoionization model calculations for the BLR. The agreement between the observed and the theoretical line correlations is very good, especially for Lyα. In particular, there is no need to invoke a luminosity-dependent covering factor, since higher luminosity discs are predicted to emit softer ionizing continuum. The alternative explanation involves line and continuum variability. While a complete theory of thin AGN discs is not yet available, the observational tests investigated in this paper may be a very promising method to look for the existence of such systems.