Polarized Optical Emission of the Blazar PKS 1222+216: Discovery of a 420 day Quasiperiodic Signal

Abstract

We report our search for quasiperiodic signals in long-term optical and γ-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 on board the Fermi Gamma-ray Space Telescope, respectively. A quasiperiodic signal, with a period of ≃420 days and a significance of >5σ, is found in the measurements of the optical linear polarization degree for the source, while no similar signals are found in the optical and γ-ray light curves covering approximately the same time period of ∼10 yr. We study the quasiperiodic 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.