The Seyfert 1 galaxy Ark 120 in 1996-2005. Temporal characteristics of the optical variability

We study the optical variability of the Seyfert galaxy Ark 120 using the autocorrelation and structure functions, as well as a periodogram analysis, based on our 1996-2005 U BV RI observations. The autocorrelation function indicates an increase in the correlation to 70% for time shifts of about 430 days. The structure function displays a decrease for these same time shifts, which is interpreted as evidence for periodic events on these timescales. The structure function indicates an increase in the variability amplitude with increasing duration of the variable process. On large time scales, the structure function can be presented as a power-law function with index 0.8-0.9, possibly indicating the presence of unstable processes in the accretion disk. Our search for periodicity in the light curves has revealed a possible period P = 430 days, which has been observed during 13 cycles, as follows from an analysis using continuum from spectral data obtained from 1988 to 2005.