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Successfull polarimetric observations with APERTIF Phased Array Feed

Submitter: Wim van Cappellen on behalf of the APERTIF team
Description: Magnetic fields are present in all different types of astronomical objects. Studying magnetic fields requires polarimetric observations. The Westerbork Synthesis Radio Telescope (WSRT) has proven to be an excellent instrument for this type of observations. In the coming years, the WSRT will be upgraded with APERTIF Phased Array Feeds (PAFs). Due to the very different nature of the APERTIF feed system, its polarimetric calibration required a novel approach. To verify the new approach, the Rotation Measure (RM) of BL Lac has been observed. Due to its large RM, the observed polarization angle varies strongly over frequency. Such measurement is very challenging since it requires a polarimetrically calibrated and stable interferometer system. Five measurements were combined to span the frequency range from 1190 to 1390 MHz. The observations were performed twice: On March 14, BL Lac was observed through an on-axis PAF beam. On March 22, BL Lac was observed through a 1 degree scanned beam, i.e. twice the half-power beamwidth of the standard WSRT!

The image above displays the observed Stokes parameters. The flux scale was not calibrated accurately, resulting in variations of Stokes I. The observed polarization angle has been derived from Stokes Q and U and plotted as function of lambda squared. Using a linear least-squares fit, an RM of -204.1 rad/m2 was found for the on-axis measurement and -204.9 rad/m2 for the 1 degree scanned beam. These results agree very well with a measurement by Ger de Bruyn using the entire WSRT. He measured an RM of -205.1 rad/m2 on April 1,2012. The slight differences can be explained by variations of the source and ionospheric conditions.

This convincingly demonstrate that the APERTIF prototype can be polarimetrically calibrated and, most importantly, that there are no major issues to perform polarimetric measurements with APERTIF phased array feeds.
Copyright: ASTRON, 2012.
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