2001A&A...374..770M

The Structure Function (SF) analysis of variable fluxes of the Seyfert galaxy NGC 4151 obtained during its extraordinary brightening in 1989-1996 in the UBVRI bands is reported. More than 1500 measurements were made in each spectral band. The logarithmic slopes of SFs for all samples of flux observations are found to be different for intranight and extranight flux variations: 0.81-0.96 and 0.44-0.60, respectively. The dependence of the SF slopes on the phase of nucleus activity was revealed. The SF slopes increased for intranight variations in accordance with the nucleus brightening and decreased for yearly variations. The processes causing the flux variations evolved during the nucleus brightening in different directions: for intranight variations the process evolved from flicker-noise to extreme shot-noise; for yearly variations - vice versa. The obtained data provide evidence in favor of the supposition that intranight and extranight variations are caused by different sources. As a result, the variable source of the NGC 4151 nucleus during the active phase in the UBVRI bands exhibited characteristics of radio loud AGNs. The highest values of logarithmic slopes of SF for variations on a time scale of years were in the range 0.8≤b≤1.1, and for 10-150 day flares they were as high as 2.2. There was flattening of the optical spectrum with increasing nucleus brightness. The brightness amplitude of the nucleus in the U band reduced to an aperture D=5'' over 7.3 years increased by a factor of more than 50. The brightness in the U band varied by a factor of more than 10 per year. The obtained characteristics of the variable source acting in the NGC 4151 nucleus from 1990-1996 do not contradict the model where the increasing nucleus brightening is caused by clouds of synchrotron radiation ejected from the nucleus during its active period.