The Variability of a Large Sample of Quasars
I. Hook, R. G. McMahon, B. J. Boyle, M. J. Irwin · Cambridge University Press eBooks · 1994
The variability properties of a sample of over 300 optically–selected quasars near the South Galactic Pole (SGP) have been studied using a series of eleven UKST B j plates at seven epochs, spanning 16 years. Quasars of high luminosity show significantly less variation than those with low luminosity. A similar, though much weaker, trend with redshift was found; lower redshift quasars varying proportionally more than high redshift quasars. The observed trends are a consequence of an intrinsic dependence of quasar variability on luminosity combined with the effects of time–dilation and have strong implications for quasar samples selected solely on variability. Introduction Variability provides a simple yet powerful means for investigating the physical processes at work in the inner regions of AGN. The primary diagnostics for optical variability are: the dependence on absolute magnitude and redshift, the timescale of variations in the quasar rest frame and the degree of coherence of individual quasar light curves — in our case taken as an ensemble. In addition to providing insight into quasar models an important feature of such a study is the ability to predict selection effects for quasar samples chosen purely on the basis of variability ( e.g ., Hawkins 1986). In this paper we summarise our method and results: a more detailed account is given in Hook et al . (1991) and Hook et al . (1992). Data The sample of quasars was taken principally from the catalogue of Hewitt & Burbidge (1989) with additional objects from other surveys.