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

• Published in: Monthly notices of the Royal Astronomical Society 2003-07, Vol.342 (4), p.1222-1240
• Main Authors: Czerny, B., Doroshenko, V. T., Nikołajuk, M., Schwarzenberg-Czerny, A., Loska, Z., Madejski, G.
• Format: Article
• Language: English
• Subjects: galaxies: active; galaxies: individual: NGC 4151; galaxies: Seyfert; instabilities; X-rays: galaxies
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1046/j.1365-8711.2003.06623.x

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 ∼10 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.