Transcript 4. Concurrent 43 and 86 GHz VLBA Polarimetry Observations of the

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)
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 2002. 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/m2 is present at ~ 1 mas.
Previous Results
Naturally weighted images of the blazar 3C273, epoch 2002.35, 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 480.0 mJy beam-1; the peak is 629.5 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 3C279, epoch 2002.35, made with the VLBA
at 43 GHz. (top) Contours of I at 50.00, 70.71, …[factors of sqrt(2)]…, 12800, and 18102
mJy beam-1; the peak is 18428 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].
Naturally weighted images of the blazar 3C273, epoch 2002.35, 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 200.0 mJy beam-1; the peak is 255.5 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 2002.35, 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 400.0 mJy beam-1; the peak is 471.7 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].
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/m2 (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 mo=0.75 represents a purely ordered field, and mu=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%.
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/m2 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/m2 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, mo=0.75 represents a purely
ordered field, and mu=0.15 represents the uniform field in the jet component.