Milliarcsecond polarization structure of the superluminal quasar 3C 273

The total intensity and linear polarization structures of the superluminal quasar 3C 273 have been determined at 5 GHz with a resolution of 2 x 10 milliarcseconds. The optically thick core is very weakly polarized (fractional polarization m <~ 0.5%). Appreciable polarized flux was detected from several superluminal components in the jet, their fractional polarization increasing systematically with distance from the core. The most distant component (C2) is highly polarized, with m near 50%, and has a magnetic field that is highly ordered and is aligned within a few degrees of the axis of the VLBI jet. Within a few milliarcseconds of the core the inferred orientation of the magnetic field varies rapidly with position along the jet. We conclude that (i) the predominantly longitudinal orientation of the magnetic field in the 3C 273 jet is established within 20 mas of the core, (ii) a highly disorganized magnetic field is the most plausible explanation for the low degree of polarization in the innermost regions of the VLBI jet, and (iii) the observed run of fractional polarization along the VLBI jet is probably due to a rapidly increasing degree of organization in the magnetic field with distance from the core. The variations in the orientation of the electric vectors near the core may be due to a significant Faraday rotation with a steep gradient, caused by ionized gas and magnetic fields in the narrow-line emission region.