Rapid Multiwaveband Polarization Variability in the Quasar PKS 0420-014: Optical Emission from the Compact Radio Jet

Abstract

An 11 day monitoring campaign in late 2005 reveals clear correlationin polarization between the optical emission and the region of theintensity peak (the ``pseudocore'') at the upstream end of the jetin 43 GHz VLBA (Very Long Baseline Array) images in the highly variablequasar PKS 0420-014. The electric-vector position angle (EVPA) ofthe pseudocore rotated by about 80° in four VLBA observations overa period of 9 days, matching the trend of the optical EVPA. In addition,the 43 GHz EVPAs agree well with the optical values when we correctthe former for Faraday rotation. Fluctuations in the polarizationat both wave bands are consistent with the variable emission arisingfrom a standing conical shock wave that compresses magnetically turbulentplasma in the ambient jet. The volume of the variable component isthe same at both wave bands, although only ~20% of the total 43 GHzemission arises from this site. The remainder of the 43 GHz fluxdensity must originate in a separate region with very low polarization.If 0420-014 is a typical case, the nonthermal optical emission fromblazars originates primarily in and near the pseudocore rather thancloser to the central engine where the flow collimates and accelerates.