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

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Abstract

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 AGNs, 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 produces 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 uctuating light curves and structure functions, V (q), by simple Monte Carlo simulations on the basis of the two models. Each resultant V (q) possesses a power-law portion, [V (q)]1@2 P qb, at short time lags (q). The two models can be distinguished by the logarithmic slope b ; b D 0.740.90 in the SB model and b D 0.410.49 in the DI model, while the observed light curves exhibit b D 0.35. Therefore, we conclude that the DI model is favored over the SB model in explaining the slopes of the observational structure function in the case of 0957]561, though this object is a radio-loud object and thus is not really a fair test for the SB model. In addition, we examine the time asymmetry of the light curves by calculating V (q) separately for the brightening and the 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.

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