Variability of accretion flow in the core of the Seyfert galaxy NGC 4151

We analyse observations of the Seyfert galaxy NGC 4151 covering 90 years in the optical band and 27 years in the 2-10 keV X-ray band. We compute the normalized power spectrum density, the structure function and the autocorrelation function for these data. The results show that the optical and X-ray variability properties are significantly different. X-ray variations are predominantly in the time-scale range of 5-1000 d. The optical variations also have a short time-scale component that may be related to X-ray variability, but the dominant effect is the long time-scale variability, with time-scales longer than similar to10 yr. We compare our results with observations of NGC 5548 and Cyg X-1. We conclude that the long time-scale variability may be caused by radiation pressure instability in the accretion disc, although the observed time-scale in NGC 4151 is by a factor of a few longer than expected. X-ray variability of this source is very similar to what is observed in Cyg X-1 but scaled with the mass of the black hole, which suggests that the radiation pressure instability does not affect the X-ray production considerably.