Concurrent 43 and 86 GHz Very Long Baseline Polarimetry of 3C 273

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Abstract
We present submilliarcsecond resolution total intensity and linear polarization VLBI images of 3C 273, using concurrent 43 and 86 GHz data taken with the Very Long Baseline Array in 2002 May. The structure seen in the innermost jet suggest that we have fortuitously caught the jet in the act of changing direction. The polarization images confirm that the core is unpolarized (fractional polarization ≤1%) at 86 GHz, but also show well ordered magnetic fields (m ~ 15%) in the inner jet, at a projected distance of 2.3 pc from the core. In this strongly polarized region, the rotation measure changes across the jet by ~4.2 × 104 rad m-2 over an angular width of about 0.3 mas. If the lack of polarization in the core is also attributed to a Faraday screen, then a rotation measure dispersion 5.2 × 104 rad m-2 must be present in or in front of that region. These are among the highest rotation measures reported so far in the nucleus of any active galaxy or quasar, and must occur outside (but probably close to) the radio emitting region. The transverse rotation measure gradient is in the same sense as that observed by Asada and coworkers and by Zavala and Taylor at greater core distances. The magnitude of the transverse gradient decreases rapidly with distance down the jet, and appears to be variable.
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