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

Abstract

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 ∼ 2200 and ∼ 4500 days appear in both continua 5100 Å and 4200 Å, as well as in the H γ emission line. Their integer ratio is nearly harmonic ∼12, 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.