A View through Faraday's Fog. II. Parsec-Scale Rotation Measures in 40 Active Galactic Nuclei

Results from a survey of the parsec-scale Faraday rotation measure (RM) properties for 40 quasars, radio galaxies, and BL Lac objects are presented. Core RMs for quasars vary from approximately 500 to several thousand rad m^-2. Quasar jets have RMs that are typically 500 rad m^-2 or less. The cores and jets of the BL Lac objects have RMs similar to those found in quasar jets. The jets of radio galaxies exhibit a range of RMs from a few hundred to almost 10,000 rad m^-2 for the jet of M87. Radio galaxy cores are generally depolarized, and only one of four radio galaxies (3C 120) has a detectable RM in the core. Several potential identities for the foreground Faraday screen are considered, and we believe the most promising candidate for all the active galactic nucleus types considered is a screen in close proximity to the jet. This constrains the path length to approximately 10 pc, and magnetic field strengths of approximately 1 μG can account for the observed RMs. For 27 out of 34 quasars and BL Lac objects, their optically thick cores have good agreement with a λ² law. This requires the different τ = 1 surfaces to have the same intrinsic polarization angle independent of the observed frequency and distance from the black hole.