Decomposing X-ray Variability in Seyfert I AGN with Principal Component Analysis

X-ray surveys of some active galactic nuclei (AGN) in the local universe show stochastic short-term variability at small amplitudes, in addition to long-term state transitions analogous to those seen in X-ray binaries. The origin of such transition X-ray variability is not fully understood, but some of them were ascribed to transient obscuration events caused by eclipsing outflow and others to X-ray coronal flares. In this work, we consider X-ray variability in a group of Seyfert I AGN, including NGC 3783, NGC 3227, Mrk 335, and NGC 4051, whose central regions are less obscured by the AGN torus. We use principal component analysis (PCA) to reduce the dimensionality of the multi-epoch X-ray time-segmented data. This gives us the PCA components that mainly cause the X-ray fluctuations. Our analysis indicates the change in the power-law emitting source, via either obscuration or flaring, is mainly responsible for transition variability. Higher-order PCA components include small variations in ionizing absorbers, which may be caused by time delays between different stratification layers in absorbing outflows, in addition to a tiny contribution (<1%) from relativistically broadened reflection emission likely from light-bending fluctuations near the event horizon, which is consistent with the fact that the supermassive black hole spin remains stable over the course of multi-epoch observations. We see that PCA decomposition could help us make X-ray variability in AGN less complicated, leading to simpler data that could be used for more accurate spectral analysis with machine learning algorithms.