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1 Introduction

The continuum variability search of active galactic nuclei (AGNs) is thought to be a useful probe for understanding their nature. Large international monitoring campaigns on some Seyfert galaxies (NGC 3783, NGC 4151, NGC 5548, NGC 7469) have produced a number of fundamental results which provide important constraints on models of the continuum source that cannot be obtained in a single observation. At present the most appropriate model is an accretion disk around a black hole.

The program of simultaneous UBVRI monitoring of nuclei of Seyfert galaxies has been ongoing at the Crimean observatory since 1989. Some results important for the modelling of the AGNs have been obtained. Two types of intranight flares in the variable flux of the NGC 1275 nucleus were revealed by Pronik et al. (1999a,b) using 1982-1994 observations. Both types of intranight flares were independent of the nucleus brightness averaged by nights. It was supposed that intranight and extranight variations are caused by different sources. Merkulova (2000) showed that two types of intranight flares acted in NGC 4151 and NGC 7469 nuclei, too. The amplitudes of intranight variations were up to 15%.

A new cycle of NGC 4151 nucleus activity beginning from 1990 was observed by Merkulova et al. (1999a,b). They reviewed the variability investigations of the NGC 4151 nucleus published by many authors and studied its extraordinary brightening between February 1990 and June 1996. According to Ulrich et al. (1997), the UV flux (1440 Å) of the new variable source was increased by a factor 5 in 1993 than it was in 1984-1985. Lyuty & Doroshenko (1999) noted that a new cycle of brightening of the NGC 4151 nucleus is distinguished from the previous cycle of activity in 1968-1983 in the UBV bands by a higher luminosity by a factor of 2. Flares of 50-100 days duration were observed both at the period of minimum in 1984-1989 and at both cycles of maxima of the nucleus activity. According to Merkulova et al. (1999) the energy distribution, obtained by UBVRI spectral band observations for flux excesses over the 1.5-2.0 years and during the flares, showed a power law form: $F_\nu \sim\nu^{\alpha_{\rm pl}}$. The increase of the power law spectral index $\alpha _{\rm pl}$ with time from the beginning to the end of the nucleus brightening indicated that there was a flattening of the optical spectrum with time. The data obtained were discussed in the framework of a model where the increase of the nucleus brightening is caused by clouds emitting synchrotron radiation ejected from the nucleus during its active period.

In this paper we present the results of a Structure Function analysis of the variable fluxes. Processes causing the nucleus variability and its evolution with time are considered.


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