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Source finding in linear polarization for LOFAR, and SKA predecessor surveys, using Faraday moments

Source finding in linear polarization for LOFAR, and SKA predecessor surveys, using Faraday moments
Source finding in linear polarization for LOFAR, and SKA predecessor surveys, using Faraday moments

The optimal source-finding strategy for linear polarization data is an unsolved problem, with many inhibitive factors imposed by the technically challenging nature of polarization observations. Such an algorithm is essential for Square Kilometre Array (SKA) pathfinder surveys, such as the Multifrequency Snapshot Sky Survey with the LOw Frequency ARray (LOFAR), as data volumes are significant enough to prohibit manual inspection. We present a new strategy of 'Faraday Moments' for source-finding in linear polarization with LOFAR, using the moments of the frequency-dependent full-Stokes data (i.e. the mean, standard deviation, skewness, and excess kurtosis). Through simulations of the sky, we find that moments can identify polarized sources with a high completeness: 98.5 per cent at a signal to noise of 5. While the method has low reliability, rotation measure (RM) synthesis can be applied per candidate source to filter out instrumental and spurious detections. This combined strategy will result in a complete and reliable catalogue of polarized sources that includes the full sensitivity of the observational bandwidth. We find that the technique can reduce the number of pixels on which RM Synthesis needs to be performed by a factor of ≈1 × 105 for source distributions anticipated with modern radio telescopes. Through tests on LOFAR data, we find that the technique works effectively in the presence of diffuse emission. Extensions of this method are directly applicable to other upcoming radio surveys such as the POlarization Sky Survey of the Universe's Magnetism with the Australia Square Kilometre Array Pathfinder, and the SKA itself.

Magnetic fields, methods: data analysis -methods: observational, Polarization, techniques: image processing, techniques: polarimetric
0035-8711
3280-3296
Farnes, J. S.
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Heald, G.
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Junklewitz, H.
64f3eda5-6017-43df-9f53-d41d981adbad
Mulcahy, D. D.
07ba7af8-66a2-470c-93d9-a4238a2ace00
Haverkorn, M.
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Van Eck, C. L.
4410875c-2595-41a1-87bf-fc95485ade80
Riseley, C. J.
f418e7cb-9709-4efd-a9a8-bfca84c7ea7b
Brentjens, M.
5b2c92e3-1a71-42cf-8b64-8167d837c751
Horellou, C.
a6e1653a-3ff6-4f17-814b-43f9995ac02c
Vacca, V.
da8a00d1-3263-4558-ba28-139199c2dac8
Jones, D. I.
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Horneffer, A.
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Paladino, R.
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Farnes, J. S.
a2a51a37-d499-4e14-9f8e-6b24b2b578a3
Heald, G.
12b841c9-c9d3-4bd7-9a69-dbf4c08294ad
Junklewitz, H.
64f3eda5-6017-43df-9f53-d41d981adbad
Mulcahy, D. D.
07ba7af8-66a2-470c-93d9-a4238a2ace00
Haverkorn, M.
b6197a6a-fda1-45aa-999a-1d124cc80367
Van Eck, C. L.
4410875c-2595-41a1-87bf-fc95485ade80
Riseley, C. J.
f418e7cb-9709-4efd-a9a8-bfca84c7ea7b
Brentjens, M.
5b2c92e3-1a71-42cf-8b64-8167d837c751
Horellou, C.
a6e1653a-3ff6-4f17-814b-43f9995ac02c
Vacca, V.
da8a00d1-3263-4558-ba28-139199c2dac8
Jones, D. I.
78bd223e-f16b-4403-a5c1-a34e77254bc6
Horneffer, A.
0a2072fa-62a3-4d0f-a179-7bdc5e26ede6
Paladino, R.
bfd4b1be-b588-49d7-acc7-a28e415d7c4e

Farnes, J. S., Heald, G., Junklewitz, H., Mulcahy, D. D., Haverkorn, M., Van Eck, C. L., Riseley, C. J., Brentjens, M., Horellou, C., Vacca, V., Jones, D. I., Horneffer, A. and Paladino, R. (2018) Source finding in linear polarization for LOFAR, and SKA predecessor surveys, using Faraday moments Monthly Notices of the Royal Astronomical Society, 474, (3), pp. 3280-3296. (doi:10.1093/mnras/stx2915).

Record type: Article

Abstract

The optimal source-finding strategy for linear polarization data is an unsolved problem, with many inhibitive factors imposed by the technically challenging nature of polarization observations. Such an algorithm is essential for Square Kilometre Array (SKA) pathfinder surveys, such as the Multifrequency Snapshot Sky Survey with the LOw Frequency ARray (LOFAR), as data volumes are significant enough to prohibit manual inspection. We present a new strategy of 'Faraday Moments' for source-finding in linear polarization with LOFAR, using the moments of the frequency-dependent full-Stokes data (i.e. the mean, standard deviation, skewness, and excess kurtosis). Through simulations of the sky, we find that moments can identify polarized sources with a high completeness: 98.5 per cent at a signal to noise of 5. While the method has low reliability, rotation measure (RM) synthesis can be applied per candidate source to filter out instrumental and spurious detections. This combined strategy will result in a complete and reliable catalogue of polarized sources that includes the full sensitivity of the observational bandwidth. We find that the technique can reduce the number of pixels on which RM Synthesis needs to be performed by a factor of ≈1 × 105 for source distributions anticipated with modern radio telescopes. Through tests on LOFAR data, we find that the technique works effectively in the presence of diffuse emission. Extensions of this method are directly applicable to other upcoming radio surveys such as the POlarization Sky Survey of the Universe's Magnetism with the Australia Square Kilometre Array Pathfinder, and the SKA itself.

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More information

Accepted/In Press date: 7 November 2017
e-pub ahead of print date: 14 November 2017
Published date: 1 March 2018
Keywords: Magnetic fields, methods: data analysis -methods: observational, Polarization, techniques: image processing, techniques: polarimetric

Identifiers

Local EPrints ID: 417456
URI: https://eprints.soton.ac.uk/id/eprint/417456
ISSN: 0035-8711
PURE UUID: 7a960316-538f-40a8-b1d8-491fa4c17712

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Date deposited: 31 Jan 2018 17:30
Last modified: 31 Jan 2018 17:30

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Contributors

Author: J. S. Farnes
Author: G. Heald
Author: H. Junklewitz
Author: D. D. Mulcahy
Author: M. Haverkorn
Author: C. L. Van Eck
Author: C. J. Riseley
Author: M. Brentjens
Author: C. Horellou
Author: V. Vacca
Author: D. I. Jones
Author: A. Horneffer
Author: R. Paladino

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