Parsec-scale Faraday rotation distribution in the BL Lac object 1803+784

We present the results of VLBI polarization observations of the BL Lac object 1803+784 made simultaneously at 22, 15, 8 and 5 GHz in 1997 April, and quasi-simultaneously at 1.6 and 5 GHz in 1998 July. These data have yielded the total intensity, polarization, spectral-index, and rotation-measure distributions on scales from less than 1 mas to about 30 mas from the core (from a projected distance of about 1-150 pc at the redshift of 1803+784). There is an enhancement in the rotation measure for the VLBI core, clearly due to plasma in the immediate vicinity of the AGN. The rotation measures 10-30 mas from the core demonstrate that the Galactic contribution to the total observed rotation measures is equal to the previously reported integrated rotation measure (-61 rad m(-2) ). After subtracting this Galactic contribution and multiplying by (1 +z )(2) , the rotation measure for the VLBI core in the source rest frame is about 500 rad m(-2), similar to the VLBI core rotation measures reported previously for BL Lac and several quasars.