Evidence for Polarized Synchrotron Components in Radio-Optical Aligned Quasars

Abstract

A tendency for the axes of double-lobed radio quasars to be aligned with the electric vectors of optical polarization in the active galactic nuclei is well-known. However, the origin of the polarization and reason behind its correlation with radio morphology is not yet established. From accurate spectropolarimetry of 7 quasars which show this alignment effect, we find that the polarization is universally confined to the continuum, and not shared by the line emission or 3000A bump. Over the observed region 4000-8000A, the spectral indices of polarized flux, P x Flambda, are consistent with uniform polarization applied to the light of the big blue bump or with synchrotron emission. However, electron scattering from an optically-thick, geometrically- thin, accretion disk is well-known to give rise to a polarization position angle that is perpendicular to the disk axis. Optical synchrotron emission akin to that shown to exist in the miniblazar 3C273 is a more attractive explanation, and supporting evidence can be found for some of the targets in the form of polarimetric variability over intervals of years. Properties of the most strongly-polarized radio-optical aligned quasars can be explained by misdirected blazar core components that have net polarizations of P~10% and provide ~10% of the total optical light.