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 ( \tau ) , by simple Monte-Carlo simulations on the basis of the two models .
Each resultant V ( \tau ) possesses a power-law portion , [ V ( \tau ) ] ^ { 1 / 2 } \propto \tau ^ { \beta } , at short time lags ( \tau ) .
The two models can be distinguished by the logarithmic slope , \beta ; \beta \sim 0.74–0.90 in the SB model and \beta \sim 0.41–0.49 in the DI model , while the observed light curves exhibit \beta \sim 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 ( \tau ) 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 .