Daily Image
30-11-2010
First detection of polarization in LOFAR imaging data
| Submitter: | Anna Scaife, Ger de Bruyn, and George Heald |
| Description: | One of the science drivers for LOFAR is the investigation of polarized radio sources. Detecting polarization is more complicated using LOFAR than with other radio telescopes: the dipoles are fixed on the ground, so the instrumental polarization is very strong and time-variable. Recently, a first big step has been made in detecting a polarized signal from PSR J0218+4232. This pulsar is known to be strongly polarized from previous WSRT observations, with a Faraday rotation measure of -61 rad/m2. The pulsar is located in the field of the radio source 3C66. Observations of this field totaling several hours of integration time were used, with a net bandwidth of 1 MHz (5 subbands) spanning a 2 MHz range centered at 129 MHz. Even within this narrow bandwidth, there were about 3 full rotations of the polarization plane. The data were calibrated using BBS, and linear polarization images were produced. The images were then processed using the technique of RM Synthesis. The output is shown in the image. On the left, an image of the polarized pulsar is shown. To the right, the amount of polarized flux (in arbitrary units) is shown as a function of the Faraday rotation measure. There is a very clear signal at a rotation measure of +60 rad/m2. This is the correct value except for the sign (positive instead of negative) - the cause of this discrepancy is understood, and results from a difference in coordinate definitions between the calibration and imaging software. Because the pulsar is detected at the correct rotation measure, we are confident that we have recovered real polarized emission with LOFAR! Several members of the Magnetism KSP are now working further on this dataset, using many more subbands. These same techniques will be used to investigate the polarized signal from many other sources in the LOFAR sky. Because the polarization properties change as the radiation from background sources passes through foreground objects like galaxies and galaxy clusters, we will be able to use these signals to learn about the magnetic fields in the foreground objects. |
| Copyright: | LOFAR |
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