Long-term optical variability properties of the Palomar-Green quasars

We present results from a monitoring programme of 42 quasars from the Palomar-Green sample. The objects were observed for seven years at the Wise Observatory, as part of a longterm effort to monitor AGN of various types. This is the most extensive programme of its kind carried out to date on a well-defined, optically selected quasar sample. The typical sampling interval is similar to 40 d. One-third of the quasars were observed at similar to 60 epochs, and the rest at similar to 30 epochs in two bands (B and R) with a photometric accuracy of similar to 0.01 mag. We present light curves for all of the sources, and discuss the sample variability properties. All of the quasars in the sample varied during the campaign with intrinsic rms amplitudes of 5 < sigma(B) < 34 per cent and 4 < sigma(R) < 26 per cent. The rms amplitude and colour for the entire sample are sigma(B) = 14 per cent, sigma(R) = 12 per cent and sigma(B-R) = 5 per cent. On time-scales of 100 to 1000 d the power spectra of the sources have a power-law shape, P(nu) proportional to nu(-gamma), with gamma approximate to 2.0 and a spread less than or similar to 0.6. At least half of the quasars, particularly those that are most variable, become bluer when they brighten, and the rest do not show this behaviour. We quantify this phenomenon, which has been observed previously mainly in Seyfert galaxies. The quasars which are most variable tend also to exhibit asymmetry in their variations, in the sense that the brightening phases last longer than the fading phases. We have searched for correlations between the measured variability properties and other parameters of the quasars, such as luminosity, redshift, radio loudness and X-ray slope. We find several new correlations, and reproduce some of the correlations reported by previous studies. Among them are an anticorrelation of variability amplitude with luminosity, a trend of the autocorrelation time-scale with luminosity, and an increase in variability amplitude with H beta equivalent width. However, all of these trends have a large scatter despite the low observational uncertainties.