Optical variability in active galactic nuclei: Starbursts or disk instabilities?

Aperiodic optical variability is a common property of active galactic nucle i (AGNs), though its physical origin is still open to question. To study th e origin of the optical-ultraviolet variability in AGNs, we compare light c urves of two models to observations of quasar 0957 + 561 in terms of a stru cture function analysis. In the starburst (SB) model, random superposition of supernovae in the nuclear starburst region produces aperiodic luminosity variations, while in the disk-instability (DI) model, variability is cause d by instabilities in the accretion disk around a supermassive black hole. We calculate fluctuating light curves and structure functions, V(tau), by s imple Monte Carlo simulations on the basis of the two models. Each resultan t V(tau) possesses a power-law portion, [V(tau)](1/2) proportional to tau(b eta), at short time lags (tau). The two models can be distinguished by the logarithmic slope beta; beta similar to 0.74-0.90 in the SB model and beta similar to 0.41-0.49 in the DI model, while the observed light curves exhib it beta similar to 0.35. Therefore, we conclude that the DI model is favore d over the SE model in explaining the slopes of the observational structure function in the case of 0957 + 561, though this object is a radio-loud obj ect and thus is not really a fair test for the SE model. In addition, we ex amine the time asymmetry of the light curves by calculating V(tau) separate ly for the brightening and the decaying phases. The two models exhibit oppo site trends of time asymmetry to some extent, although the present observat ion is not long enough to test this prediction.