Linear Polarization Imaging with Very Long Baseline Interferometry at High Frequencies
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Abstract
We describe a new method to self-calibrate the instrumental polarization characteristics in polarimetric very long baseline interferometry (VLBI) observations. The method is especially useful at high frequencies where no suitable calibrator sources with known polarization properties are available. In a test experiment with the Very Long Baseline Array (VLBA), the feed polarization parameters (D factors) were calibrated to within 0.2% rms using a compact continuum source. This made possible measuring the fractional polarization with a maximum accuracy of 0.15%. Higher calibration accuracy could be obtained with H_2_O-maser sources. We demonstrate that the electric vector orientation can be calibrated internally with the VLBA at 22 GHz. Therefore, VLBP observations without calibrator sources are possible at this frequency. We applied the method to VLBA observations of three active galactic nuclei (AGN) and imaged the linear polarization structure of AGN for the first time at 22 GHz. In 3C 345 and 3C 279 the inferred magnetic field is predominantly perpendicular in the jet. In 3C 273 the magnetic field is approximately longitudinal within 2 mas of the core: at larger distances the field is random. The core polarization is low in all three quasars (<3%), whereas the jet components are up to 30% polarized. The polarization properties of 3C 345 and 3C 279 are largely consistent with a general model based on strong plane-perpendicular shocks in the jet plasma; the properties of 3C 273 cannot be easily explained by simple shock models.
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