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G.B. Taylor (NRAO)
Multi-frequency polarimetry with the Very Long Baseline Array (VLBA) telescope has revealed absolute Faraday Rotation Measures (RMs) in excess of 1000 rad m-2 in the central regions of 7 out of 8 strong quasars studied (e.g. 3C\,273, 3C\,279, 3C\,395, etc). Beyond a projected distance of ~20 h-1 pc, however, the jets of all seven sources are found to have |RM| < 100 rad m-2. Such sharp RM gradients cannot be produced by cluster or galactic-scale magnetic fields, but rather must be the result of magnetic fields organized over the central 1--100 pc. The direct detection of high RMs in these quasar cores can explain the low fractional core polarizations usually observed in quasars at centimeter wavelengths as the result of irregularities in the Faraday screen on scales smaller than the telescope beam. Variability in the RM of the core has been seen in 3C\,279 between observations taken 1.5 years apart, indicating that the Faraday screen changes on that timescale, or that the projected superluminal motion of the inner jet components samples a new location in the screen with time. Either way, these changes in the Faraday screen may help to explain the dramatic variability in core polarization properties displayed by quasars.
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