Abstract  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.

Abstract  We have studied the variability of the Hβ line and the adjacent continuum in the spectrum of the Seyfert galaxy Ark 120, based on spectral observations of the galaxy’s
nucleus obtained in the Crimea in 1992–2005, supplemented by published data for 1988–1996. Irregular variability on various
timescales (years to days) can be accompanied by periodic brightness variations in both the continuum and the Hβ line, with a period of P ∼ 430 days and an amplitude of Δm ∼ 0.2
^m
in the continuum, which were traced for more than 13 cycles. In total, in 1988–2005, the flux variations in the line lag
those in the continuum by 55 ± 9 days if calculated from the peak of the cross correlation function, or by 72 ± 7 days, if
calculated from the centroid of the CCF. The delay is correlated with the continuum brightness, increasing when the continuum
flux increases. The Hβ line profiles indicate both a high degree of diversity and the presence of features that recur after various extended time
intervals. Analysis of the evolution of the differences between each individual normalized line profile and the mean normalized
profile indicates systematic motion of excesses relative to the average profile from negative to positive radial velocities.
In contrast, parts of the Hβ line with low radiation relative to the mean normalized profile evolve in the opposite direction (from the red to the blue
Hβ wing). This pattern is also typical for the rotating broad-line region, if this region has the form of a disk. The rotation
period exceeds 9000–10000 days, or 25–27 years. The size of the broad-line region calculated form this period corresponds
to a reverberation time of no fewer than 30 days, consistent with the results of cross-correlation analysis.

Abstract  Photometry of the nucleus of the Seyfert 1 galaxy Ark 120 acquired in 1996–2005 with telescopes of the Crimean Astrophysical
Observatory and the Sternberg Astronomical Institute’s Crimean Laboratory is used to construct and analyze the object’s U BV RI variability. The U BV RI variations have different timescales, and the variability amplitude grows towards shorter wavelengths. Changes in the color
indices of the variable source appear to be due to variations in the opacity of gas at temperatures T > 15 000 K. The spectral energy distribution (SED) of the rapidly varying component is bluer than the SED of the slowly-varying
component. The rapid variations are due to instabilities in the inner part of the accretion disk where, according to current
ideas, the continuum is formed. Based on the peak of the cross-correlation function, the V RI variations lag the B variations by 1.4 to 5.6 days, with the lag increasing systematically from V to I. The lag calculated from the centroid of the cross-correlation function is from 2.8 days for V to 10.5 days for I. We demonstrate that the wavelength dependence of the lag is well approximated by the function τ ∝ λ^4/3, characteristic of optically thick accretion disks.