QSO variability: probing the Starburst model

Abstract

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 as 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 days. 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.