Evidence for Accretion Disks in Highly Polarized Quasars

We present UBVRI polarimetry and photometry of 11 highly polarized quasars (HPQs). Three of these HPQs (0420-014, 1156+295, and 3C 454.3) clearly show decreasing linear polarization toward shorter wavelengths. We model the observed optical continua of these HPQs as a combination of polarized synchrotron emission, unpolarized emission from the broad-line region, and an unpolarized flat spectral component that may be optically thick thermal emission from an accretion disk. Marginal evidence for such a thermal component is also seen in the data for 1510-089 and 2345-167. Rough upper limits for the brightness of a thermal component can be placed on five other HPQs. We find a range in luminosity of at least a factor of 10 for the HPQ thermal components. The near-ultraviolet luminosities of the HPQ thermal components derived from our models fall near the average luminosity of the quasars in the PG survey. Only the presence of a bright synchrotron component seems to distinguish some HPQs from low-polarization, optically selected QSOs at optical and near-UV wavelengths.