Time-Variable Faraday Rotation Measures of 3C 273 and 3C 279

Multifrequency polarimetry with the VLBA confirms the previously reportedtime-varying Faraday rotation measure (RM) in the quasar 3C-279. Variability inthe RM and electric vector position angle (EVPA) of the jet component (C4) isseen making it an unreliable absolute EVPA calibrator. 3C-273 is also shown tovary its RM structure on 1.5 year time-scales. Variation in the RM propertiesof quasars may result from a Faraday screen which changes on time-scales of afew years, or from the motion of jet components which sample spatial variationsin the screen. A new component emerging from the core of 3C-279 appears to bestarting to sample such a spatial variation. Future monitoring of thiscomponent and its RM properties is suggested as a diagnostic of the narrow lineregion in 3C-279. We also present a new method of EVPA calibration using theVLA Monitoring Program.Comment: Accepted to ApJ Letters. 12 pages, 5 figure