Optical monitoring of quasars - I. Variability

We present an analysis of quasar variability from data collected during a p hotometric monitoring of 50 objects carried out at CNPq/Laboratorio Naciona l de Astrofisica, Brazil, between 1993 March and 1996 July. A distinctive f eature of this survey is its photometric accuracy, similar to 0.02 V mag, a chieved through differential photometry with CCD detectors, which allows th e detection of faint levels of variability. We find that the relative varia bility, delta = sigma/L, observed in the V band is anticorrelated with both luminosity and redshift, although we have no means of discovering the domi nant relation, given the strong coupling between luminosity and redshift fo r the objects in our sample. We introduce a model for the dependence of qua sar variability on frequency that is consistent with multiwavelength observ ations of the nuclear variability of the Seyfert galaxy NGC 4151. We show t hat correcting the observed variability for this effect slightly increases the significance of the trends of variability with luminosity and redshift. Assuming that variability depends only on the luminosity, we show that the corrected variability is anticorrelated with luminosity and is in good agr eement with predictions of a simple Poissonian model. The energy derived fo r the hypothetical guises, similar to 10(50) erg, agrees well with those ob tained in other studies. We also find that the radio-loud objects in our sa mple tend to be more variable than the radio-quiet ones, for all luminositi es and redshifts.