Optical monitoring of luminous AGN - I. Radio-loud quasars

Abstract

We describe six years of a Band R monitoring program of a sample of 44 radio-loud quasars. The observations were performed at the Wise Observatory, Cerro Tololo Interamerican Observatory and McDonald Observatory with a mean sampling of about 4-8 times a year and a typical accuracy of 0.02-0.03 mag. We show the B, R and B -R light-curves of all sources and discuss the sample properties in terms of variability amplitude, luminosity, continuum shape, radio structure and variability time-scale. All objects in our sample have varied during the monitoring campaign. In 15 per cent of the sources the variability is smaller than 0.2 mag and in 45 per cent between 0.2 and 0.4 mag. The amplitude of the variations does not depend on redshift or luminosity. Furthermore, there is no difference, in this respect, between core-dominated and lobe-dominated sources. Our sampling is adequate to calculate the autocorrelation function of 34 sources in the sample and to use it to define the typical variability time-scale. We find that the rest-frame variability time is inversely correlated with the source luminosity. However, external effects caused by the sampling period and the frequency can strongly influence this conclusion and we suggest ways to try to eliminate such biases. If correct, the inverse correlation with luminosity can dominate any cosmological (1 +z) dependence.