Testing the X-ray variability of active galactic nuclei with the   nonlinear prediction method
  The analysis of eight EXOSAT X-ray lightcurves of six active galactic nuclei by nonlinear prediction methods indicates that the observed short time-scale variability is truly stochastic and is not caused by deterministic chaos. This result favors X-ray emission models with mutliple centers of production of hot, possibly relativistic electrons. 
B. Czerny (Copernicus Astronomical Center, Warsaw), H.J. Lehto (Turku   University Observatory, Tuorla)