QSO variability: Probing the starburst model

The consistency of the starburst model for AGN is tested using the optical variability observed in large data bases of QSOs, Theoretical predictions for the variability-luminosity relationship and structure function are presented and compared with observations. If QSOs follow a variability-wavelength relation like that observed in nearby AGN, the model proves successful in reproducing the main characteristics of optical variability, The wavelength dependence (1) flattens the (otherwise monochromatic) Poissonian variability-luminosity relationship; and (2) decreases the asymptotic value of the structure function, which reveals that the elementary pulse driving the variations would have a characteristic time-scale of 85-280 d. The upper limit is consistent with the time-scale found in nearby Seyfert galaxies, Shorter values of this time-scale are expected if the metallicity of high-redshift objects is high, as recent observations indicate. If distant QSOs do not follow the variability-wavelength dependence observed in Seyfert nuclei and nearby QSOs, the characteristic pulse of variation needs to be much faster in order to reproduce the variability-luminosity relationship, but then the single-parametric model explored in this work predicts a more rapidly rising structure function than that inferred from the data.