Periodicity in the continua and broad line curves of a quasar E1821+643

Here we present an in-depth analysis of the periodicity of the continua and broad emission lines of a quasar E1821+ 643. We applied non-parametric composite models, the linear sum of stationary and non-stationary Gaussian processes, and quantified contribution of their periodic parts. We found important qualitative differences among the three periodic signals. Periods of similar to 2200 and similar to 4500 days appear in both continua 5100 angstrom and 4200 angstrom, as well as in the H gamma emission line. Their integer ratio is nearly harmonic similar to 1/2, suggesting the same physical origin. We discuss the nature of these periods, proposing that the system of two objects in dynamical interaction can be origin of two largest periods.