Faraday rotation as a probe of the physical conditions near relativistic jets

Multi-frequency polarimetry with the VLBA can be used to produce Faraday Rotation Measure (RM) maps of the central parsecs of the hosts of relativistic jets. Quasars and radio galaxies have been shown to have RMs of several 1000 rad m ². Thus, knowledge of the Faraday Rotation in a source is required to recover the intrinsic magnetic field direction in relativistic jets viewed through a large Faraday depth. As the RM is a measure of the magnetic field weighted by the electron density, estimates of the magnetic field strength and topology can be made in close proximity to the jet. We will present results from a survey of 40 quasars, radio galaxies, and BL Lac objects and discuss their RM properties and the nature of the foreground screen responsible for the observed Faraday Rotation.