On the nature of X-ray variability in Ark 564

Abstract

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.

XMM-Newton observation of the NLS1 galaxy Ark 564

“…When combined, these two results support the idea of a small black hole mass, and hence high accretion rate, in Ark 564. Finally, Gliozzi et al (2002) found no statistically significant indications of non-stationarity in the ASCA light curves. Furthermore, using nonlinear techniques they were able to demonstrate that the source behaves differently in the high and low flux states.…”

Probing the photoionised outflow in the NLS1 Arakelian 564: AnXMM-Newtonview

“…We perform spectral analysis on the stacked spectrum to determine the ionisation and dynamical structure of the outflow. Ark 564, like several other NLS1s, is already known to be a highly variable source, best illustrated during the month-long ASCA observation by Gliozzi et al (2002), when it displayed non-linear behaviour. Given the timespan of our data, we look for long-term variability of the source and the resulting gas response.…”

“…Unlike continuous monitoring surveys, which allow analysis of evenly spaced time intervals, e.g. Gliozzi et al (2002), here we have time-averaged observations taken over a decade. However, the 2011 data were taken every six days from May 2011 to July 2011 and so here we use this set to look for correlations.…”

A search for chaos in the optical light curve of a blazar: W2R 1926+42

“…Indications of the possible fractal nature of the light curves of a few other blazars were found by Marchenko & Hagen-Thorn (1997); however, again -huge missing parts of the light curves were replaced by means of interpolation. Nonlinear (and perhaps chaotic) variability has been found by Gliozzi et al (2002) in a 10 000-point X-ray light curve of Akn 564, a radio-quiet AGN observed by Ginga satellite. Chaotic signatures in ∼20 000-point-long light curves in the X-ray binary GRS 1915+105 (Misra et al 2004) was also reported.…”