Quasar discs – I. The Baldwin effect

The optical ‘bump’ in quasar continua may be due to thermal processes that are only weakly related to the energy producing mechanism at other wavelengths. A likely possibility is emission from the surface of an optically thick accretion disc. Here it is suggested that such discs may be geometrically thin, having different inclinations to the line-of-sight that result in a range of apparent optical luminosity for a given intrinsic disc luminosity. The total line emission is more isotropic so a fixed line luminosity is associated with an observed range of continuum luminosity, and thus a range of line equivalent width. This provides a natural (although not the only) explanation to the observed anticorrelation of EW (C IV) with the continuum luminosity at 1550 Å (the ‘Baldwin effect’). Analysis of a radio-selected sample of quasars strengthens this suggestion. This sample, as well as others, shows two apparently contradictory correlations of lines and continuum that have been misunderstood in the past. The problem is resolved when the disc inclination is taken into account. It is shown that such samples do not have constant line or continuum luminosities and should not be used to deduce cosmological parameters. The inclination of the disc may also explain some of the observed correlations of optical, radio and X-ray emission in quasars, in particular the slope of the $${L}_{\text{op}}/{L}_{\text{X}}$$ relation. X-ray, radio and IR emission in quasars is probably not originating in the same place as the optical emission but this must be further investigated. Several other ideas, such as beaming and continuum variability, are also discussed, as well as the special case of Seyfert galaxies.