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J. M. Attridge (MIT Haystack Observatory), J. F. C. Wardle (Brandeis University), D. C. Homan (NRAO), T. P. Krichbaum (MPfIR), A. Greve (IRAM)
Observations of 3C\,273 and 3C\,279 taken in April 2000 with the Coordinated Millimeter VLBI Array (CMVA) resulted in the first 86~GHz VLBI total intensity (Stokes I) and linear polarization images of any source (Attridge 2001). As Faraday effects are proportional to \lambda2, 86~GHz data should be less affected by Faraday rotation and depolarization than lower frequency data. The four antenna (FD, LA, PT, KP-12m) data defy the expectation that increased levels of linear polarization will be found in the cores of quasars at high frequencies, for both 3C\,273 and 3C\,279 display low levels of polarization in their cores at 86~GHz. In fact, 3C\,273 displays {\em no} measurable linear polarization within a limit of 1%.
The lack of significant linear polarization in the cores of 3C\,273 and 3C\,279, as well as the high rotation measures (>1000~rad m-2) derived for the cores of many quasars (including 3C\,273 and 3C\,279) by Taylor (1998, 2000) suggest the presence of parsec-sized Faraday screens with organized magnetic fields near the quasar cores. If 3C\,273's core is depolarized by Faraday effects alone, the dispersion of the rotation measure is \gtrsim90000~rad m-2 in the core. Alternately, depolarization can be explained in the case where 89% of the magnetic energy is contained in a tangled magnetic field.
A second epoch of 86~GHz CMVA polarization observations is presented. Data were taken in April 2001, almost exactly one year after the previous epoch, and with twice as many antennas. In addition to confirming the previous results, the superior resolution provided by the additional antennas allows further restriction of the sizes of the Faraday screens in the cores of 3C\,273 and 3C\,279.
CMVA research at Haystack Observatory is sponsored by the National Science Foundation.