Bayesian Approach to Find a Long-Term Trend in Erratic Polarization Variations Observed in Blazars

TL;DR

This paper introduces a Bayesian method to extract long-term polarization trends from erratic blazar data, revealing systematic behaviors in some objects and complex short-term variations in others.

Contribution

A novel Bayesian approach for separating long-term polarization trends from short-term variations in blazars, validated with artificial data and applied to multiple sources.

Findings

Successfully separated long-term polarization components in two blazars.

Detected no long-term trend in S5 0716+714, indicating complex short-term behavior.

Revealed that erratic polarization variations can be due to underlying long-term components.

Abstract

We developed a method to separate a long-term trend from observed temporal variations of polarization in blazars using a Bayesian approach. The temporal variation of the polarization vector is apparently erratic in most blazars, while several objects occasionally exhibited systematic variations, for example, an increase of the polarization degree associated with a flare of the total flux. We assume that the observed polarization vector is a superposition of distinct two components, a long-term trend and a short-term variation component responsible for short flares. Our Bayesian model estimates the long-term trend which satisfies the condition that the total flux correlates with the polarized flux of the short-term component. We demonstrate that assumed long-term polarization components are successfully separated by the Bayesian model for artificial data. We applied this method to photopolarimetric data of OJ 287, S5 0716+714, and S2 0109+224. Simple and systematic long-term trends were obtained in OJ 287 and S2 0109+224, while no such a trend was identified in S5 0716+714. We propose that the apparently erratic variations of polarization in OJ 287 and S2 0109+224 are due to the presence of the long-term polarization component. The behavior of polarization in S5 0716+714 during our observation period implies the presence of a number of polarization components having a quite short time-scale of variations.