1999MNRAS.305..309G

We present a power spectral analysis of the 0.1–2 keV X-ray light curves of NGC 4051. Two long ROSAT observations have been analysed and the power spectra are shown to have significantly different shapes, indicating that the source is not statistically stationary on time-scales of ∼ 1 year. There is also strong evidence that the light curve is non-linear at certain epochs, which means that the power spectra do not adequately represent all the information contained in the light curve. The implications of these discoveries are far reaching and fundamental to the study of the X-ray light curves of active galactic nuclei. The two currently popular models for X-ray variability (multi-time-scale shot noise and rotating flares on a disc) are linear, and so cannot fully describe the data: however, models in which the variability events are correlated rather than random may be more successful in describing the observed light curves.