The Qso Variability-luminosity-redshift Relation

The relationship between variability, luminosity and redshift in the South Galactic Pole QSO sample is examined in an eeort to disentangle the eeects of luminosity and redshift in the amplitude of the optical variations. The anticorrelation between variability and luminosity found by other authors is connrmed. Our analysis also supports claims that variability increases with redshift, most likely due to an anticorrelation between variability and wavelength. In particular, our parametric ts show that the QSO variability-wavelength relation is consistent with that observed in low-luminosity nearby active galactic nuclei. The results are used to constrain Poissonian-type models. We nd that if QSO variability results from a random superposition of pulses, then the individual events must have B-band energies between 10 50 and a few times 10 51 erg and timescales of 2 yr. Generalized Poissonian models in which the pulse energy and lifetime scale with luminosity are also discussed.