Ultraviolet variability of NGC 4151: A study using principal component analysis

Abstract

In an attempt at a decomposition of the different sources of variation in the IUE spectra of NGC 4151, we have carried out a principal component analysis of the spectra, seeking the number and form of the significant linearly independent spectra that are needed to reconstruct the data within the errors. Methods are outlined with particular emphasis on the effect of noise. Simulations are presented which aid interpretation of the results. We verify the independent existence of about ten variable components as identified previously using physical intuition, and demonstrate that they account for most of the statistically significant variations in the available data set. No simple physical interpretation can be made for most of the linearly independent components, suggesting that (except for estimating the number of independent variable components) principal component analysis has a fairly limited application to the analysis of spectroscopic variability of Active Galactic Nuclei.