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

## TL;DR

This paper analyzes the periodicity in the continuum and emission lines of quasar E1821+643, revealing multiple harmonic periods likely caused by interacting objects within the system.

## Contribution

It introduces a novel application of non-parametric composite models to identify and analyze multiple periodic signals in quasar data.

## Key findings

- Detected two main periods of approximately 2200 and 4500 days.
- Found the periods have a nearly harmonic ratio of 1/2.
- Proposed that interacting objects could explain the observed periodicities.

## 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 $\sim 2200$ and $\sim 4500$ days appear in both continua 5100 \AA \, and 4200 \AA \,, as well as in the H$\gamma$ emission line. Their integer ratio is nearly harmonic $\sim \frac{1}{2}$, suggesting the same physical origin. We discuss the nature of these periods, proposing that the system of two objects in dynamically interaction can be origin of two largest periods.