Probing Magnetic Fields using the Giant Metrewave Radio Telescope

Farnes, J. S.; Green, D. A.; Kantharia, N. G.

doi:10.48550/arxiv.1309.4646

Cited by 3 publications

(2 citation statements)

References 12 publications

(17 reference statements)

Supporting

Mentioning

Contrasting

Order By: Relevance

“…For these real data, many sources are visible in the Q and U skew images, which are displayed on the same scales, and show a larger skew in Stokes U. This skew is also associated with M51 itself, which Farnes, Green, & Kantharia (2013) and Mulcahy et al (2014) showed to be unpolarized at low radio frequencies. The increased skew is therefore most likely the result of polarization leakage.…”

Section: Application To Real Lofar Datamentioning

confidence: 83%

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

Farnes

Heald

Junklewitz

et al. 2017

Monthly Notices of the Royal Astronomical Society

View full text Add to dashboard Cite

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 (MSSS) 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 frequencydependent 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% 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 × 10 5 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 (POSSUM) with the Australia Square Kilometre Array Pathfinder (ASKAP), and the SKA itself.

show abstract

Section: Application To Real Lofar Datamentioning

confidence: 83%

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

Farnes

Heald

Junklewitz

et al. 2017

Monthly Notices of the Royal Astronomical Society

View full text Add to dashboard Cite

show abstract

“…For Faraday tomography of extragalactic objects, such as nearby galaxies, low-frequency data has not been as successful. Mulcahy et al (2014) and Farnes et al (2013) used observations from LOFAR at 150 MHz and the Giant Meterwave Radio Telescope at 610 MHz respectively to search for polarized emission from M51, but did not detect significant polarization. Faraday tomography has been successfully applied to nearby galaxies at higher frequencies (e.g.…”

Section: Recent Observationsmentioning

confidence: 99%

The Power of Low Frequencies: Faraday Tomography in the sub-GHz regime

Eck¹

2018

Preprint

View full text Add to dashboard Cite

Faraday tomography, the study of the distribution of extended polarized emission by strength of Faraday rotation, is a powerful tool for studying magnetic fields in the interstellar medium of our Galaxy and nearby galaxies. The strong frequency dependence of Faraday rotation results in very different observational strengths and limitations for different frequency regimes. I discuss the role these effects take in Faraday tomography below 1 GHz, emphasizing the 100-200 MHz band observed by the Low Frequency Array and the Murchison Widefield Array. With that theoretical context, I review recent Faraday tomography results in this frequency regime, and discuss expectations for future observations.

show abstract

Constraining Direction-Dependent Instrumental Polarisation: A New Technique for Polarisation Angle Calibration

Farnes¹

2014

Preprint

View full text Add to dashboard Cite

Direction-dependent instrumental polarisation introduces wide-field polarimetric aberrations and limits the dynamic range of low-frequency interferometric images. We therefore provide a detailed two-dimensional analysis of the Giant Metrewave Radio Telescope (GMRT) primary beam in full-Stokes at 325 MHz and 610 MHz. We find that the directional dependence is essentially independent of the feed and is dominated by the curvature of the dishes reflecting mesh. The developed beam models are used to reduce wide-field instrumental polarisation in 610 MHz observations by subtracting the expected response from the uv-data itself. Furthermore, a new technique for polarisation angle calibration is presented that allows for calibration using an unpolarised source and therefore can be implemented at arbitrarily low observational frequencies. This technique has the advantage that it calibrates the polarisation angle independently of ionospheric Faraday rotation and source variability. It also removes the need for known polarised sources on the sky -which are scarce at low frequencies. We use the technique to retrieve the Rotation Measure of pulsar B1937+21 at 325 MHz, finding it to be consistent with previous independent measurements. An extended version of this method may be useful for verifying the calibration of other interferometers intended for polarimetric surveys.

show abstract

Probing Magnetic Fields using the Giant Metrewave Radio Telescope

Cited by 3 publications

References 12 publications

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 Power of Low Frequencies: Faraday Tomography in the sub-GHz regime

Constraining Direction-Dependent Instrumental Polarisation: A New Technique for Polarisation Angle Calibration

Contact Info

Product

Resources

About