ABSTRACT April 2000 CMVA observations of the sources 3C273 and 3C279 resulted in the first VLBI total intensity and linear polarization images of any source at 86 GHz (Attridge 2001, ApJL, 553, L31). Here we present new 43 and 86 GHz observations of the two sources collected concurrently in May The VLBA was equipped with seven 86 GHz receivers and ten 43 GHz receivers, effectively yielding similar array resolution at the two frequencies. In addition to confirming the previous results at 86 GHz, the improved resolution and availability of spectral information allows investigation into possible causes of the depolarization seen in quasar cores at high frequencies. An observed differential Faraday rotation in 3C273 implies a Faraday screen of at least 35,000 rad/m 2 is present at ~ 1 mas. Summary of New Results While the overall EVPA calibration at 86 GHz has not been performed, comparison of the polarization angles between 86 GHz and 43 GHz in 3C 273 is still of considerable interest. We find that the two polarized components at 86 GHz need different amounts of rotation to align them to our 43 GHz image. This difference is ~1 radian, and implies a strong differential Faraday screen with rotation measures of at least 35,000 rad/m 2 in the source frame. The core of 3C273 continues to be unpolarized Spectral index maps reveal both 3C273 and 3C279 to be optically thin at these frequencies (not surprising, they are in between outbursts!) Observed fractional polarization values are similar to the previous data set Typical D-term value of ~3% at 43 GHz, and ~10% at 86 GHz Myers/Taylor VLA/VLBA polarization calibration database utilized to calibrate EVPA at 43 GHz Performing similar calculations to those done with previous data as shown to left: Looking at Faraday depolarization in 3C273, the standard deviation of the RM for the core of 3C273 is  RM ~80,000 rad/m 2 using m=0.15 and =0.0035m Looking at the ordering of the magnetic field in 3C273, the resulting fraction (f) of magnetic energy in the tangled component is ~93%. Again, m o =0.75 represents a purely ordered field, and m u =0.15 represents the uniform field in the jet component. Concurrent 43 and 86 GHz VLBA Polarimetry Observations of the Quasars 3C273 and 3C279 J. M. Attridge (MIT Haystack Observatory), J. F. C. Wardle (Brandeis University), D. C. Homan (NRAO), R. B. Phillips (MIT Haystack Observatory) Why VLBP at 86 GHz? Probe near jet’s origin where shocks or shear initially order B field Faraday rotation and depolarization reduced at high frequencies (  2) Theory predicts up to 75% fractional polarization (m) may exist in optically thin regions BUT at cm, only see modest levels of m (<10%) in the cores of AGN Large RMs found in quasar cores suggest pc sized Faraday screens with organized B fields will be found near the cores (Lister & Smith. 2000, ApJ, 541, 66) Faraday Depolarization in 3C273 If we assume the lack of polarization in 3C273’s core is due to Faraday depolarization alone, and we have a Gaussian distribution of RMs over a finely spaced grid (Burn 1966, MNRAS, 133, 67), then in the case of 3C273: m=0.1 =0.0035m Then  RM ~90,000 rad/m 2 (the standard deviation of the RM) for the core of 3C273. Ordering of the Magnetic Field Assuming the B field in the unshocked jet fluid contains a tangled component PLUS a uniform parallel component then the degree of order of the B field is the fraction (f) of magnetic energy in the tangled component (Burn 1966; Wardle et al. 1994, ApJ, 437, 122): Here m o =0.75 represents a purely ordered field, and m u =0.10 represents the uniform field in component C1. Solving for  and then f, the resulting fraction of magnetic energy in the tangled component in 3C273 is ~90%. Note A ~90  EVPA calibration constant has been assumed and applied. Typical D-term value of ~12%. Previous Results Naturally weighted images of the blazar 3C273, epoch , made with the VLBA at 43 GHz. (top) Contours of I at 15.00, 21.21, …[factors of sqrt(2)]…, 2715, and 3840 mJy beam -1 ; the peak is 4547 mJy beam -1, and the restoring beam shown in the lower left is 0.48 x 0.23 mas at φ=-1.35°. (middle) Contours of p at 15.00, 21.21, …[factors of sqrt(2)]…, 339.4, and mJy beam -1 ; the peak is mJY beam -1. The ticks show the orientation χ of the electric field in the source. (bottom) Contours of m as in I, but in steps of [factor of 2]. Naturally weighted images of the blazar 3C273, epoch , made with the VLBA at 86 GHz. (top) Contours of I at 25.00, 35.36, …[factors of sqrt(2)]…, 1131, and 1600 mJy beam -1 ; the peak is 1815 mJy beam -1, and the restoring beam shown in the lower left is 0.46 x 0.24 mas at φ=-15.3°. (middle) Contours of p at 25.00, 35.36, …[factors of sqrt(2)]…, 141.4, and mJy beam -1 ; the peak is mJY beam -1. The ticks show the uncalibrated orientation χ of the electric field in the source. (bottom) Contours of m as in I, but in steps of [factor of 2]. Naturally weighted images of the blazar 3C279, epoch , made with the VLBA at 86 GHz. (top) Contours of I at 20.00, 28.28, …[factors of sqrt(2)]…, 5120, and 7241 mJy beam -1 ; the peak is 7524 mJy beam -1, and the restoring beam shown in the lower left is 0.38 x 0.23 mas at φ=-9.21°. (middle) Contours of p at 100.0, 141.4, …[factors of sqrt(2)]…, 282.8, and mJy beam -1 ; the peak is mJY beam -1. The ticks show the uncalibrated orientation χ of the electric field in the source. (bottom) Contours of m as in I, but in steps of [factor of 2]. Naturally weighted images of the blazar 3C279, epoch , made with the VLBA at 43 GHz. (top) Contours of I at 50.00, 70.71, …[factors of sqrt(2)]…, 12800, and mJy beam -1 ; the peak is mJy beam -1, and the restoring beam shown in the lower left is 0.53 x 0.22 mas at φ=-0.85°. (middle) Contours of p at 50.00, 70.71, …[factors of sqrt(2)]…, 800.0, and 1131 mJy beam -1 ; the peak is 1153 mJY beam -1. The ticks show the orientation χ of the electric field in the source. (bottom) Contours of m as in I, but in steps of [factor of 2].