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 ∼2200${\sim}2200$ and ∼4500${\sim}4500$ days appear in both continua 5100 Å and 4200 Å, as well as in the Hγ$\mathrm{H}\upgamma$ emission line. Their integer ratio is nearly harmonic ∼12${\sim}\frac{1}{2}$, 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.