Polarization Properties of Blazars

Abstract Blazars are an extreme subclass of active galactic nuclei showing rapid and large-amplitude variability, high and variable polarization, strong γ-ray emissions, or superluminal motion, etc. If a strong beaming effect is the reason for the extreme observation properties, then one would expect that these properties would be correlated with each other. In this paper, we compiled a sample of 341 blazars with optical polarization and other data, and investigated the correlation between the optical polarization and multi-wavelength luminosity, as well as apparent velocity. Our analysis comes to the following conclusions: (1) Polarization and observed luminosity (at radio, optical, and γ-ray bands) are all positively correlated in BL Lacertae (BL Lac) objects. (2) A γ-ray source or a higher apparent velocity source tend to have a higher polarization. (3) The polarization distributions of BL Lac objects and flat spectrum radio quasars (FSRQs) come from different parent distribution, with the polarization in Bl Lac objects being higher than that in FSRQs.