Optical Variability in Active Galactic Nuclei: Starbursts or Disk Instabilities?

Aperiodic optical variability is a common property of Active Galactic Nuclei (AGNs), though its physical origin is still open to question. To study the origin of the optical – ultraviolet variability in AGN, we compare light curves of two models to observations of quasar 0957+561 in terms of a structure function analysis. In the starburst (SB) model, random superposition of supernovae in the nuclear starburst region produce aperiodic luminosity variations, while in the disk-instability (DI) model, variability is caused by instabilities in the accretion disk around a supermassive black hole. We calculate fluctuating light curves and structure functions, V (τ), by simple Monte-Carlo simulations on the basis of the two models. Each resultant V (τ) possesses a power-law portion, [V (τ)] ∝ τ, at short time lags (τ). The two models can be distinguished by the logarithmic slope, β; β ∼ 0.74–0.90 in the SB model and β ∼ 0.41–0.49 in the DI model, while the observed light curves exhibit β ∼ 0.35. Therefore, we conclude that the DI model is favored over the SB model to explain the slopes of the observational structure function, in the case of 0957+561, though this object is a radio-loud object and thus not really a fair test for the SB model. In addition, we examine the time-asymmetry of the light curves by calculating V (τ) separately for brightening and decaying phases. The two models exhibit opposite trends of time-asymmetry to some extent, although the present observation is not long enough to test this prediction. Subject headings: accretion, accretion disks — instability — active galactic nuclei