The optical variability of QSOs

The long-term variability of a sample of 180 optically selected QSOs in the field of the Selected Area 94 has been studied. The relations between variability and luminosity and between variability and redshift have been investigated by means of statistical estimators that are ''robust'' and allow at the same time to eliminate the influence of the measurement errors. A comparison is carried out with the results of two other samples of QSOs, in the fields of the South Galactic Pole (Hook et al. 1994) and of the Selected Area 57 (Trevese et al. 1994). Merging the three samples provides a total of 486 QSOs. The analysis in the QSOs rest frame of both the ensemble structure function (SF) and the individual variability indices show that: 1) a negative correlation between variability and luminosity is clearly present, in the sense that more luminous QSOs show less variability; 2) a significant positive correlation exists between variability and redshift; 3) such correlations may be equally well parameterized either with a model in which the timescale of the variability is fixed for all the QSOs (tau similar to 2.4 yr), while the amplitude linearly increases with the absolute magnitude and redshift, or with a model in which the timescale of the variability linearly depends on the absolute magnitude and the amplitude is only a function of the redshift. The same analysis carried out in the observer's frame provides the following results: 1) there is a negative correlation between variability and luminosity; 2) the timescale of variability does not depend significantly either on the absolute magnitude or on the redshift; 3) the ensemble structure function is well represented by a parameterization in which, with a fixed timescale of about 5.5 yr, the amplitude linearly increases with the absolute magnitude; 4) although the general behaviour of the SF does not show a systematic variation of the timescale and/or amplitude with redshift, if we examine the average variability index for objects with -25 > M(B) > -27, we find that below redshift 1 quasars are significantly less variable than at higher redshift. The implications in terms of the black-hole, starburst and microlensing models are briefly discussed