Anisotropic optical and X-ray emission in quasars

IT is of fundamental importance in the study of quasars to understand whether or not their continuum emission is isotropic, because this affects the overall energy budget and can also help to distinguish between the different models proposed for their central regions1. The reported small range of emission-line equivalent widths in quasars have earlier been used to argue against a relativistically beamed or anisotropic continuum2,3. Here we use recent spectroscopic observations of 46 radio-loud quasars to show that the equivalent width of the [O III] 5,007-A emission line is strongly anticorrelated with the degree of radio core prominence R, which has been found to be a statistical indicator of orientation relative to the line of sight4–6. This observed anticorrelation shows that the quasar continuum is anisotropic and provides important evidence in favour of relativistic-beaming unified schemes4,7,8. We also show that the strength of the [O III] emission line is independent of R, consistent with unified schemes.