5 Conclusions

Using the Structure Function analysis of the UBVRI variability of the nucleus of the Seyfert galaxy NGC 4151 during its extraordinary brightening from 1989-1996 the following results were obtained:

1. We argued that the processes of intranight and extranight flux variations are caused by different sources. The processes in both cases evolved from the beginning to the end of the brightening but in different directions: for intranight variations the process evolved from flicker-noise to shot-noise, for yearly variations - vice versa. The evolution of the intranight process from flicker-noise to shot-noise was in accordance with the increase of the nucleus brightness. At the same time, the increase of nucleus brightness was accompanied by the evolution of extranight process from shot-noise to flicker-noise.

2. The source causing the nucleus NGC 4151 brightening was a source of high ultraviolet emission and of a high degree of variations. The increase of the nucleus brightening in an aperture D=5'' over 7.3 years in the V band was by a factor of 9 and in the U band it was by factor of more than 50.

3. The current opinion, that SF slopes of optical flux variations on time-scales of months and years are about 0.3-0.5 and that they are always less than the SF slopes for radio flux variations in AGNs, is not correct. We found that the slopes depend on the phase of nucleus activity. Our data showed that during the active phase the slopes were equal to 1.0 and during the flares they increased to 2.0.

4. The slopes of the spectral energy distribution in the UBVRI region ( $\alpha _{\rm pl}$) are found to be in anticorrelation with the SF slopes "b'' of variable flux. One possible explanation of this fact is that the increase of the nucleus brightness is caused by clouds emitting synchrotron radiation ejected from the nucleus during its active period.

Acknowledgements

We thank Dr. V. I. Pronik for helpful discussion and Dr. S. G. Sergeev for the SF program package.

The research described in this publication was made possible in part by Award No. UP1-2116 of the U.S. Civilian Research & Development Foundation for the Independent States of the Former Soviet Union (CRDF).