Polarized Optical Emission of the Blazar PKS 1222+216: Discovery of A 420-day Quasi-Periodic Signal

TL;DR

This study discovers a significant 420-day quasi-periodic signal in the optical polarization of blazar PKS 1222+216, highlighting the potential of polarimetry to probe jet magnetic field configurations.

Contribution

First detection of a 420-day quasi-periodic polarization signal in PKS 1222+216, demonstrating polarimetry's effectiveness in studying blazar jet magnetic structures.

Findings

A >5σ significance quasi-periodic polarization signal at 420 days.

No similar periodicity found in optical or gamma-ray flux light curves.

Helical jet model successfully explains the observed polarization variations.

Abstract

We report our search for quasi-periodic signals in long-term optical and $\gamma$-ray data for the blazar PKS~1222+216, where the data are from the Steward Observatory blazar monitoring program and the all-sky survey with the Large Area Telescope onboard the {\it Fermi Gamma-ray Space Telescope}, respectively. A quasi-periodic signal, with a period of $\simeq$420\,days and a significance of $>5\sigma$, is found in the measurements of the optical linear polarization degree for the source, while no similar signals are found in the optical and $\gamma$-ray light curves covering approximately the same time period of $\sim$10\,yr. We study the quasi-periodic variations by applying a helical jet model and find that the model can provide a good explanation. This work shows that polarimetry can be a powerful tool for revealing the physical properties, in particular the configuration of the magnetic fields of jets from galactic supermassive black holes.