Daily Image
31-07-2014
LOFAR images of polarized emission from our Galaxy
| Submitter: | Vibor Jelic |
| Description: | The presence of polarized foregrounds is a serious complication for epoch of reionization (EoR) experiments. To avoid the leakage of polarized emission into total intensity, which can depend on frequency, we need to calibrate the instrumental polarization across the field of view to a small fraction of 1%. Given the rather uniform levels of the polarized intensity, the polarity of Stokes Q,U is a good indicator of the spatial variations of the plane of polarization. The figure shows LOFAR HBA (150 MHz) wide-field images of polarized emission from our Galaxy (the ELIAS-N1 region) in: The RM-synthesis images are given at Faraday depths of -1.5, -0.5, +0.5, and +1.5 rad/m2 (different rows). They are 5.7deg x 5.7deg in size with a resolution (PSF) of 3.4 arcmin x 3.1 arcmin. The fractional polarization at 150 MHz, expressed as a percentage of the total intensity, amounts to 1.5%. There is no indication of diffuse emission in total intensity in the interferometric data, in line with results at higher frequencies. The average brightness temperature of the detected emission is 4 K. This is much more than was anticipated on the basis of earlier observations with other telescopes. For comparison, we have also used the WSRT at 350 MHz to image the same region. The LOFAR and WSRT images show a similar complex morphology at comparable brightness levels, but their spatial correlation is very low. The different polarized patterns observed at 150 MHz and 350 MHz are consistent with different source distributions along the line of sight wring in a variety of Faraday thin regions of emission. The wide range and richness of the detected morphological features demonstrate once more the power of low frequency polarimetry with LOFAR for studying the interstellar medium at high Galactic latitudes. Its wide frequency range, high angular resolution, and high sensitivity make LOFAR an exquisite instrument for studying Galactic polarized emission at a resolution of 1-2 rad m^-2 in Faraday depth. For more details: http://arxiv.org/abs/1407.2093 |
| Copyright: | Vibor Jelic |
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