A spectrophotometric study of BQS quasars

Flux-calibrated spectra of 39 low-redshift (z < 0.5) quasars from the Bright Quasar Survey (BQS) are presented. We examine the relations between optical continuum luminosity, L_cont, and the line luminosities of broad Hβ, L_Hβ and narrow [O III] 5007, L_{[O III]}, in these objects and in a further 37 that have been previously observed. A statistical analysis is used to show that L_Hβ is a better measure than L_cont of the photoionizing luminosity at the NLR, in the light of which, we investigate the possible causes of scatter in the luminosity relationships and study the effects of an orientation-dependent L_cont on the rest-frame emission-line equivalent widths, EW_Hβ and EW_{[O III]}. Several constraints result, for example that scatter in the $${L}_{H\beta}-{L}_\text{cont}$$ relation is not due to a range in broad-line covering factor, and is inconsistent with one simple orientation scheme unless objects with very high disc inclination angles ($$\theta \gtrsim\,{75}^{^{\circ}}$$) are excluded by the BQS selection criteria. We argue that some explanations of the ‘Baldwin Effect’ – an effect not seen in our study – rely on the existence of high-θ objects in the samples studied. We also demonstrate the existence of a correlation between the full width at half maximum of the Hβ line and L_Hβ, providing evidence for a gravitationally bound Hβ-emitting region and a scaling $$M_\text {central}\underset{\thicksim}{\propto} L$$.