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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 theSteward Observatory blazar monitoring program and the all-sky survey with theLarge Area Telescope onboard the {\it Fermi Gamma-ray Space Telescope},respectively. A quasi-periodic signal, with a period of $\simeq$420\,days and asignificance of $>5\sigma$, is found in the measurements of the optical linearpolarization degree for the source, while no similar signals are found in theoptical and $\gamma$-ray light curves covering approximately the same timeperiod of $\sim$10\,yr. We study the quasi-periodic variations by applying ahelical jet model and find that the model can provide a good explanation. Thiswork shows that polarimetry can be a powerful tool for revealing the physicalproperties, in particular the configuration of the magnetic fields of jets fromgalactic supermassive black holes.

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