FIRST RESULTS FROM THE LAS-CAMPANAS QSO BRIGHTNESS MONITORING PROGRAM

We have examined optical brightness changes in 151 luminous, moderate- redshift QSOs with unknown variability properties. Short exposures wer e obtained using CCD detectors on the 40 inch (102 cm) Swope Telescope at Las Campanas Observatory. Some of the observations were taken as e arly as 1987, with the bulk of them beginning in 1990 and continuing t hrough the beginning of 1994. Observations of individual QSOs were typ ically taken several times per year in V and/or Gunn r filters. Bright ness variations have been determined relative to stars contained in an approximate to 6 arcmin(2) held. We investigated the characteristics of the brightness variations using the ensemble structure function for three QSO object classes: radio quiet QSOs with broad absorption line s (BALs) in their spectra, radio quiet QSOs without BALs, and radio-lo ud quasars that systematically lack BALs. We find that approximate to 2/3 of the entire sample show very quiescent variations, with changes typically less than 0.15 mag yr(-1) (proper time), corresponding to in dividual structure function slopes less than 0.0225 mag(2) yr(-1). We have designated these objects optically weak variables (OWVs), while t he remainder of the sample with larger variations are designated optic ally strong variables (OSVs). Over the time of the observations, we we re able to find only subtle statistical differences in the variability properties of radio quiet non-BAL and BAL QSOs. Although there is a s light tendency for the rate of change of continuum brightness to be la rger in the radio-quiet non-BAL QSOs, to first order in both object cl asses the mean rate of change is similar and the characteristic timesc ale of variation appears to be greater than 1.2 yr (proper time). Cons istent with the finding of slightly larger rates of change in radio-qu iet non-BAL QSOs, we also find that the fraction of OSVs is marginally larger in the non-BAL QSOs in comparison to the BAL QSOs. However, th e optical variability properties of the radio-loud quasars are clearly different. For the radio-loud quasars, while their rate of change in brightness is similar to changes observed in radio-quiet QSOs, their c haracteristic timescale of variation is statistically shorter, approxi mate to 0.6-0.8 yr (proper time). Collectively, these results have imp lications for models of QSO geometry and future methods of conducting QSO optical variability surveys. The results also illustrate the limit ations of methods for discovering QSOs on the basis of optical variabi lity alone. of the entire (proper time).