Complex Variability of AGN and XRBs Revealed by Recurrence Analysis

The light curves in multiple bandwidths of many black hole X-ray binaries (XRBs) and active galactic nuclei (AGN) are complex and resist traditional time series analysis techniques (Phillipson et al. 2018). Recurrence plots (RPs) and their quantitative analysis (RQA) were developed to study recurrences of dynamical trajectories in phase space. The structures present in RPs correlate to specific types of dynamical behavior in a system, including periodic, stochastic and chaotic motion, which are features of particular interest in the light curves of XRBs and AGN. The quantification of the structures in RPs has been applied to many fields, most recently in the characterization of stochastic behavior and nonlinear instabilities in the X-ray variability of microquasars, and the stability of orbits of terrestrial planets in the habitable zones of extrasolar systems. We apply the methods of RPs and their quantification to the three-year optical light curve of a Kepler AGN, which has been identified with a possible low-frequency quasi-periodic oscillation (Smith et al. 2018) corresponding to a temporal period of 44 days, and the X-ray light curves of four well-studied black hole XRBs from the RXTE, MAXI, and NuSTAR observatories. We provide evidence for traces of periodic, stochastic, and nonlinear, possibly chaotic, behavior in these light curves and compare to the results from our recent radiation-hydrodynamic simulations.