  We present results of recurrence analysis of 46 active galactic nuclei (AGN)
using light curves from the 157-month catalog of the Swift Burst Alert
Telescope (BAT) in the 14-150 keV band. We generate recurrence plots and
compute recurrence plot metrics for each object. We use the surrogate data
method to compare all derived recurrence-based quantities to three sets of
stochastic light curves with identical power spectrum, flux distribution, or
both, in order to determine the presence of determinism, non-linearity,
entropy, and non-stationarity. We compare these quantities with known physical
characteristics of each system, such as black hole mass, Eddington ratio, and
bolometric luminosity, radio loudness, obscuration, and spectroscopic type. We
find that almost all AGN in this sample exhibit substantial higher-order modes
of variability than is contained in the power spectrum, with approximately half
exhibiting nonlinear or non-stationary behavior. We find that Type 2 AGN are
more likely to contain deterministic variability than Type 1 AGN while the same
distinction is not found between obscured and unobscured AGN. The complexity of
variability among Type 1 AGN is anticorrelated with Eddington ratio, while no
relationship is found among Type 2 AGN. The connections between the recurrence
properties and AGN class suggest that hard X-ray emission is a probe of
distinct accretion processes among classes of AGN, which supports
interpretations of changing-look AGN and challenges the traditional unification
model that classifies AGN only on viewing angle.
