Spectropolarimetry with the Allen Telescope Array: Faraday Rotation toward Bright Polarized Radio Galaxies

We have observed 37 bright, polarized radio sources with the Allen Telescope Array (ATA) to present a novel analysis of their Faraday rotation properties. Each source was observed during the commissioning phase with 2 to 4 100-MHz bands at frequencies ranging from 1 to 2 GHz. These observations demonstrate how the continuous frequency coverage of the ATA's log-periodic receiver can be applied to the study of Faraday rotation measures (RMs). We use RM synthesis to show that wide-bandwidth data can find multiple RM components toward a single source. Roughly a quarter of the sources studied have extra RM components with high confidence (brighter than ~40 mJy), when observing with a RM resolution of roughly 100 rad/m2. These extra components contribute 10%-70% of the total polarized flux. This is the first time multiple RM components have been identified in a large sample of point sources. For our observing configuration, these extra RM components bias the measurement of the peak RM by 10-15 rad/m2 ; more generally, the peak RM cannot be determined more precisely than the RM beam size. Comparing our 1-2 GHz RM spectra to VLBA polarimetric maps shows both techniques can identify complicated Faraday structures in the sources. However, the RM values and fractional polarization are generally smaller at lower frequencies than in the higher-frequency VLBA maps. With a few exceptions, the RMs from this work are consistent with that of earlier, narrow-bandwidth, all-sky surveys. This work also describes the polarimetry calibration procedure and that on-axis ATA observations of linear polarization can be calibrated to an accuracy of 0.2% of Stokes I. Future research directions include studying the time-dependent RM structure in Active Galactic Nuclei (AGNs) and enabling accurate, wide-area RM surveys to test models of Galactic and extragalactic magnetic fields.