14 POLARIMETRY

In VLBA polarimetric observations, sub-bands are assigned in pairs to opposite hands of circular polarization at each frequency. Typical ``impurities'' of the antenna feeds are about 3% for the center of most VLBA bands and degrade toward the band edges and away from the pointing center in the image plane. Without any polarization calibration, an unpolarized source will appear to be polarized at the 2% level. Furthermore, without calibration of the RCP-LCP phase difference, the polarization angle is undetermined. With a modest investment of time spent on calibrators and some increased effort in the calibration process, the instrumental polarization can be reduced to less than 0.5%.

To permit calibration of the feed impurities (sometime also called ``leakage'' or ``D-terms''), VLBA users should include observations of a strong ($\sim$1 Jy) calibration source, preferably one with little structure. This source should be observed during at least 5 scans covering a wide range ($>$ 100 degrees) of parallactic angle, with each scan lasting for several minutes. The electric vector polarization angle (EVPA) of the calibrator will appear to rotate in the sky with parallactic angle while the instrumental contribution stays constant. Some popular calibrator choices are J0555+3948=DA193 and J1407+2827=OQ208, although either or both may be inappropriate for a given frequency or an assigned observing time. Fortunately, many calibrators satisfying the above criteria are available.

A viable alternative approach to measuring polarization leakage is to use an unpolarized calibrator source. This can be done with a single scan.

To set the absolute EVPA on the sky, it is necessary to determine the phase difference between RCP and LCP. For VLBA users at frequencies of 5 GHz and above, the best method for EVPA calibration is to observe one or two of the compact sources that are being monitored with the EVLA¹; see Taylor & Myers (2000) and http://www.vla.nrao.edu/astro/calib/polar . At 1.6 GHz it may be preferable to observe a source with a stable, long-lived jet component with known polarization properties. At frequencies of 5 GHz and below one can use J0521+1638=3C138 (Cotton et al. 1997a), J1331+3030=3C286 (Cotton et al. 1997b), J1829+4844=3C380 (Taylor 1998), or J1902+3159=3C395 (Taylor 2000). At 8 GHz and above one may use J1256-0547=3C279 (Taylor 1998) or J2136+0041=2134+004 (Taylor 2000), although beware that some of these jet components do change on timescales of months to years. It will be necessary to image the EVPA calibrator in Stokes $I$, $Q$ and $U$ to determine the appropriate correction to apply. Thus it is recommended to obtain 2 to 4 scans, each scan lasting at least 3 minutes, over as wide a range in hour angle as is practical.

To permit calibration of the RCP-LCP delays, VLBA users should include a 2-minute observation of a very strong ($\sim$10 Jy) calibration source. While 3C279 is a good choice for this delay calibration, any very strong fringe-finder will suffice.

Post-processing steps include amplitude calibration; fringe-fitting; solving for the RCP-LCP delay; self-calibration and Stokes $I$ image formation; instrumental polarization calibration; setting the absolute position angle of electric vectors on the sky; and correction for ionospheric Faraday rotation, if necessary (Cotton 1995b, 1999a; Kemball 1999). All these post-processing steps can currently be done in AIPS (Section 20.1), as can the polarization self-calibration technique described by Leppänen, Zensus, & Diamond (1995).