AGN X-ray light curves - shot noise or low-dimensional attractor ?

Abstract

We use correlation integrals to determine whether the X-ray variability of AGN, as seen in EXOSAT observations, arises from chaotic systems. We distinguish three characteristic types of behaviour. We also analyse a number of simulated light curves, to investigate the poorly understood limitations to the correlation integral method caused by noise and sampling. This enables us to identify three out of eight sources as being consistent with white noise, and four sources as being consistent with a shot-noise model with a range of shot amplitudes or decay times. The light curve of only one source, NGC 4051, appears to harbour a low- dimensional attractor of dimension <= 4.5, implying that the variability may be chaotic. Because NGC 4051 is intrinsically a factor of ~10^2^ fainter than the other sources, the time-scales we sample could reflect global (chaotic) behaviour in that source, whereas the same time-scale would reflect local (shot/white noise) behaviour in the other sources.