The miniblazar in 3C 273

The compact core in 3C 273 has been studied for over 2 years using optical photometry and accurate polarimetry. A wealth of polarization behavior is observed at a level of under 1%. Highly significant variations in the normalized Stokes parameters Q/I and U/I occur on time scales of days to years, and the changes appear stochastic on the Q/I versus U/I plane. The peak polarization is almost 2%, and the amplitude of the polarization variability increases toward longer wavelengths. Position angle rotations with wavelength are small. 3C 273 differs from other low-polarization active galactic nuclei (AGNs), which do not show large variations in the amount or orientation of linear polarization. The effects of dust scattering cannot explain the observations. A simple stochastic model with highly polarized subunits flaring and fading also fails to account for the data. The results indicate that 3C 273 harbors a weak continuum component having all the characteristic properties of blazars. This miniblazar component (p ~ 10'%-20%) is probably diluted by light typical of a normal low-polarization quasar. We predict that other compact radio sources with low optical polarization (p < 3%) will also be found to harbor miniblazars. In general, the detectability of optical blazar emission is a function of the compactness of the radio source. In addition to the variable component in 3C 273, a small and stable polarization of only 0.3% is seen in the ultraviolet, at position angle of 60^deg^ +/- 10^deg^. This quiescent polarization has a weak wavelength dependence, as in many low-polarization quasars, and is probably caused by electron scattering. The detection of variable polarization in 3C 273 strengthens the link between superluminal motion at radio wavelengths and blazar activity at optical wavelengths. At the time of maximum nuclear polarization, the position angle represents a magnetic field perpendicular to the jet axis. The location of the quiescent ultraviolet polarization is uncertain, but may arise from an accretion disk close to the power source. The miniblazar in 3C 273 lends support to the "unified" beaming models with both a beamed and an isotropic component for the optical and radio emission.