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Astron. Astrophys. 321, 123-128 (1997)

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1. Introduction

The detailed study of statistically well-defined samples of optically selected QSOs is beginning to pin down their ensemble variability properties. An anti-correlation between variability and luminosity has been observed, such that more luminous QSOs show a smaller amplitude of the variations (Hook 1994, Cristiani et al. 1996). It starts also to emerge from the statistical noise a positive correlation between the amplitude of the variability and the redshift (Giallongo et al. 1991, Trevese et al. 1994, Cristiani et al. 1996, Cid Fernandes et al. 1996). This latter dependence may be interpreted as an anti-correlation between amplitude of the variability and wavelength in the individual objects (Cristiani et al. 1996, Cid Fernandes et al. 1996, Di Clemente et al. 1996), as suggested by observations of relatively small samples of objects (Perola et al. 1982, Edelson et al. 1990, Kinney et al. 1991, Paltani & Courvoisier 1994). Both properties bear important implications for the understanding of the AGN energy generation mechanism and distinguishing among the competing variability models based on the black-hole (Rees 1984), starburst (Terlevich et al. 1992) and microlensing (Hawkins 1993, 1996) scenarios. At the same time they are central for the quantification of the biases affecting samples selected on the sole basis of variability (Veron & Hawkins 1995) or on the basis of multicolor data, if the photometry was carried out at different epochs in the various wavebands (e.g. Warren et al. 1991).

However the variability-redshift correlation could be due to an evolutionary effect, in the sense of a higher activity in the early phases of the AGN life. The only way to remove such an uncertainty is to check directly the variability properties of individual objects, by observing them simultaneously in two or more wavebands. For this reason we have investigated the R-band variability of the SA94 QSO sample, the same that in Cristiani et al. 1996 (in the following Paper I) had been studied in the B band.

We report in Sect. 2 the description of the SA94 QSO sample and the photometric material used to investigate its variability; in Sect. 3 the ensemble structure function is introduced as a statistical measure of the variability properties and the detected trends are further analysed with the aid of other statistical indices; the results and implications are discussed in Sect. 4.

Throughout the paper we have assumed cosmological constants [FORMULA] km sec-1 Mpc-1, [FORMULA].

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998