Complex Variability of Kepler AGN Revealed by Recurrence Analysis
  The advent of new time domain surveys and the imminent increase in
astronomical data expose the shortcomings in traditional time series analysis
(such as power spectra analysis) in characterising the abundantly varied,
complex and stochastic light curves of Active Galactic Nuclei (AGN). Recent
applications of novel methods from non-linear dynamics have shown promise in
characterising higher modes of variability and time-scales in AGN. Recurrence
analysis in particular can provide complementary information about
characteristic time-scales revealed by other methods, as well as probe the
nature of the underlying physics in these objects. Recurrence analysis was
developed to study the recurrences of dynamical trajectories in phase space,
which can be constructed from one-dimensional time series such as light curves.
We apply the methods of recurrence analysis to two optical light curves of
Kepler-monitored AGN. We confirm the detection and period of an optical
quasi-periodic oscillation in one AGN, and confirm multiple other time-scales
recovered from other methods ranging from 5 days to 60 days in both objects. We
detect regions in the light curves that deviate from regularity, provide
evidence of determinism and non-linearity in the mechanisms underlying one
light curve (KIC 9650712), and determine a linear stochastic process recovers
the dominant variability in the other light curve (Zwicky 229--015). We discuss
possible underlying processes driving the dynamics of the light curves and
their diverse classes of variability.
