THE VARIABILITY OF OPTICALLY SELECTED QUASARS

The variability properties of a sample of approximately 300 optically selected quasars near the South Galactic Pole (SGP) have been studied over a baseline of 16 years using a series of twelve UK Schmidt Telescope B(J) plates at seven epochs. In this sample there is a correlation between variability and both quasar luminosity and redshift, at greater than the 99 per cent significance level. Quasars of high luminosity show significantly less variability than those with low luminosity, the rms variation ranging from about 0.25 mag at M(B) = -23 to 0.1 mag at M(B) = -28. The observed trends can be explained by an intrinsic dependence of quasar variability on luminosity combined with the effects of time dilation, and have implications for quasar samples selected by variability or multicolour techniques. The change in the slope of the n(m) relation at the break can be less pronounced in variability-selected samples. Our results can be used to test models for the origin and reprocessing of the continuum emission in quasars.