A View Through Faraday's Fog 2: Parsec Scale Rotation Measures in 40 AGN
NAVAL OBSERVATORY FLAGSTAFF AZ
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Results from a survey of the parsec scale Faraday rotation measure properties for 40 quasars, radio galaxies and BL Lac objects are presented. Core rotation measures for quasars vary from approximately 500 to several thousand radsq m. Quasar jets have rotation measures which are typically 500 rad radsq m or less. The cores and jets of the BL Lac objects have rotation measures similar to those found in quasar jets. The jets of radio galaxies exhibit a range of rotation measures from a few hundred radsq m to almost 10,000 radsq m for the jet of M87. Radio galaxy cores are generally depolarized, and only one of four radio galaxies 3C 120 has a detectable rotation measure in the core. Several potential identities for the foreground Faraday screen are considered and we believe the most promising candidate for all the AGN types considered is a screen in close proximity to the jet. This constrains the path length to approximately 10 parsecs, and magnetic field strengths of approximately 1 micro Gauss can account for the observed rotation measures. For 27 out of 34 quasars and BL Lacs their optically thick cores have good agreement to a Wavelength exp 2 law. This requires the different tau 1 surfaces to have the same intrinsic polarization angle independent of frequency and distance from the black hole.