On the nature of X-ray variability in Ark 564

¹ George Mason University, Department of Physics and Astronomy, MS 3F3, 4400 University Dr., Fairfax, VA 22030-4444, USA
² Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
³ Centre for Interdisciplinary Plasma Science, Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
⁴ Foundation for Research and Technology–Hellas, 711 10 Heraklion, Crete, Greece
⁵ Physics Department, University of Crete, 710 03 Heraklion, Crete, Greece
⁶ Cosmic Radiation Laboratory, RIKEN, 2-1 Hirosawa, Wako–shi, Saitama 351-0198, Japan

Corresponding author: M. Gliozzi, mario@physics.gmu.edu

Received: 16 October 2001
Accepted: 11 June 2002

Abstract

We use data from a recent long ASCA observation of the Narrow Line Seyfert 1 galaxy Ark 564 to investigate in detail its timing properties. We show that a thorough analysis of the time series, employing techniques not generally applied to AGN light curves, can provide useful information to characterize the engines of these powerful sources. We searched for signs of non–stationarity in the data, but did not find strong evidences for it. We find that the process causing the variability is very likely nonlinear, suggesting that variability models based on many active regions, as the shot noise model, may not be applicable to Ark 564. The complex light curve can be viewed, for a limited range of time scales (as indicated by the breaks in the structure and power density spectrum), as a fractal object with non–trivial fractal dimension and statistical self–similarity. Finally, using a nonlinear statistic based on the scaling index as a tool to discriminate time series, we demonstrate that the high and low count rate states, which are indistinguishable on the basis of their autocorrelation, structure and probability density functions, are intrinsically different, with the high state characterized by higher complexity.