A new activity cycle of NGC 4151: Evidence for the existence of an accretion disk

New UBV observations of the nucleus of the Seyfert galaxy NGC4151 are prese nted. The light curve in forty years is analyzed. It is noted that after th e extended photometric minimum of 1984-1989, when the contribution of the v ariable source to the total flux was nearly zero, a new activity cycle bega n. Between 1989 and 1996, a significant brightening was observed: the U bri ghtness through a 27." 5 aperture rose by 2.(m)5. Based on multiaperture ph otometry, we allow for the stellar emission from the underlying galaxy and separate the variable source in the nucleus. All published UBV observations are also used for the analysis. A colorimetric analysis of the variable so urce shows that the variable source at the new active phase (1990-1998, cyc le B) differed from that operated in 1968-1983 (cycle A) by its characteris tics. First and foremost, the luminosity of the variable source at the maxi mum of cycle B was twice its luminosity at the maximum of cycle A, while th e color indices were the same. This strongly suggests that the variable sou rce increased in size. The color characteristics of the variable source in the two cycles are generally the same, but the dispersion of B-V in cycle A is a factor of 3 or 4 larger. In general, the color indices of the variabl e sources in cycles A and B change with luminosity in a similar manner. At minimum and maximum light, they corresponded to a blackbody radiation at te mperatures of similar to 8000 K and similar to 30000 K, respectively; a con siderable ultraviolet excess was observed in the latter case. The disappear ance of the old variable source and the appearance of a new variable source with slightly different characteristics in a mere similar to 10 years can be explained by changes in the accretion disk around the supermassive compa ct object (a black hole?) in the galactic nucleus.