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Differential Faraday rotation measured through beam mapping

Submitter: George Heald
Description: The "beam mapping" technique for measuring the shapes of LOFAR's station beams has been recently developed (see the daily image from 29 Feb 2012). The beam mapping that has been done so far has clearly shown that the beam properties of many stations are very close to the modeled behavior, while other stations need some work. The technique relies on a good calibration of each station, in 225 simultaneous directions centered on a bright reference source (Cygnus A, so far). But what happens if effects other than the beams dominate the calibration solutions?

An interesting example of that situation is shown in today's animation. In these beam maps, based on data from 24 February 2012, it was immediately obvious that the gain response of some remote stations was in the off-diagonal elements, and especially at low frequencies. The core stations involved in the observation (not shown) do not demonstrate this behavior - their gain response is all in the diagonal elements, where it should be.

It seemed a strong possibility that this effect was caused by Differential Faraday Rotation (DFR; see recent daily image by Ger de Bruyn). To show that this explanation is consistent with the data, the image on the right shows the rotation of the gain response from diagonal elements into off-diagonal elements, as a function of wavelength squared. The clear linear dependence is a strong indication that this is indeed DFR. For this station, RS208, the derived value of DFR is 0.017 rad/m2, relative to the central part of the array. Several other (distant) remote stations show the same behavior: RS307 (0.015 rad/m2), RS508 (0.031 rad/m2), and RS509 (DFR too high to accurately measure using this technique). Note that the (separate) observations at each of the displayed frequencies were not simultaneous, but all took place within the same 15-minute window.
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