High-Frequency VLBA polarimetry of blazars

We are engaged in a long-term project to monitor the polarized intensity structure of 15 blazars (including quasars, BL Lac objects, and radio galaxies) at 7 mm with the VLBA at bimonthly or, in some cases, more frequent intervals. The polarization is observed contemporaneously at 230 GHz with the JCMT and at optical wavelengths, in total flux at BIMA, and, in 5 cases, in X-rays with RXTE. This follows polarization snapshots of many gamma -ray bright blazars. In some parts of the jets in many sources, the magnetic field lies either parallel or perpendicular to the jet, but in many components it is oriented at an oblique angle to the jet axis. In some cases, there is good correspondence between the polarization at 1.35 mm and that of one of the most compact components in the jet at 7 mm, but in other sources it appears that the polarized component at 1.35 mm does not appear on the 7 mm image, probably because of opacity effects and/or Faraday rotation/depolarization. In the 2 radio galaxies (3C 120 and 3C 111), and in 3C 273, the core is unpolarized. The polarization at 7 mm changes very rapidly --- on timescales less than 2 months --- in several sources. Many sources contain a mixture of stationary and superluminally moving knots. The VLBA by itself (i.e., without the addition of the VLA) produces high-quality polarized intensity images at 7 mm. However, in order to monitor variability of the polarized structure in many blazars, the time sampling must be at least bimonthly. This research is supported in part by the National Science Foundation through grant AST-9802941 and by various NASA guest investigator grants (RXTE and CGRO)