Investigating non-linear and stochastic hard X-ray variability of active galactic nuclei using recurrence analysis

We present results of recurrence analysis of 46 active galactic nuclei (AGNs) using light curves from the 157-month catalogue 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 AGNs in this sample exhibit substantial higher order modes of variability than is contained in the power spectrum, with approximately half exhibiting non-linear or non-stationary behaviour. We find that Type 2 AGNs are more likely to contain deterministic variability than Type 1 AGNs, while the same distinction is not found between obscured and unobscured AGNs. The complexity of variability among Type 1 AGNs is anticorrelated with Eddington ratio, while no relationship is found among Type 2 AGNs. 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 AGNs, which supports interpretations of changing-look AGNs and challenges the traditional unification model that classifies AGNs only on viewing angle.