Mid-Frequency Aperture Array Research Articles

Calibratability of mid-frequency aperture arrays with self-holography

The stations of the mid-frequency aperture array (MFAA) envisaged for the second phase of the Square Kilometer Array (SKA) are expected to consist of order 103 to 104 receive paths. This will make calibration procedures based on the array covariance matrix computationally expensive. Recently, self-holography (SH) was proposed to overcome this issue. This method assumes that the signal from the chosen calibration source can be sufficiently well isolated. We study the signal-to-interference ratio (SIR) of the most suitable calibration source over a sidereal day for a representative MFAA station located on the SKA site in South Africa. We find that an SIR of 20 dB or higher is achievable over (most of) a sidereal day. This is well above the 11.5 dB needed to keep the beamformer efficiency after calibration above 99%. We therefore conclude that SH is a promising candidate for station calibration of the MFAA stations.

On the front-end design of mid-frequency aperture array for square kilometre array

Complementary to the conventional dish radio telescopes, aperture arrays provide a technically attractive approach to achieve a large field of view and flexibility in observational parameters e.g. Sky area vs. bandwidth. Designs of both aperture array elements and overall geometry for the SKA Mid Frequency Aperture Array are presented here, together with resulting trade-offs. The paper reports the latest developments of global efforts on the front-end design of Mid-Frequency Aperture Array, not attempting to make technology selections, as the priority of sciences and the time for implementing Mid-Frequency Aperture Array is yet to be fully confirmed. Different on-going front-end solutions are introduced, particularly crossed ring antenna array with a planar structure is explored in more detail as it is less known in the community. Key performances of the candidate front-end technologies are addressed by examining the prototypes. The objective of the collaborative study is to increase technology readiness for implementation of Mid-Frequency Aperture Array in the future.

Measurements of the cosmological evolution of magnetic fields with the Square Kilometre Array

We investigate the potential of the Square Kilometre Array (SKA) for measuring the magnetic fields in clusters of galaxies via Faraday rotation of background polarised sources. [...] We find that about 10 per cent of the sky is covered by a significant extragalactic Faraday screen. Most of it has rotation measures between 10 and 100 rad/m/m. We argue that the cluster centres should have up to about 5000 rad/m/m. We show that the proposed mid frequency aperture array of the SKA as well as the lowest band of the SKA dish array are well suited to make measurements for most of these rotation measure values, typically requiring a signal-to-noise of ten. We calculate the spacing of sources forming a grid for the purpose of measuring foreground rotation measures: it reaches a spacing of 36 arcsec for a 100 hour SKA observation per field. We also calculate the statistics for background RM measurements in clusters of galaxies. We find that a first phase of the SKA would allow us to take stacking experiments out to high redshifts (>1), and provide improved magnetic field structure measurements for individual nearby clusters. The full SKA aperture array would be able to make very detailed magnetic field structure measurements of clusters with more than 100 background sources per cluster up to a redshift of 0.5 and more than 1000 background sources per cluster for nearby clusters, and could for reasonable assumptions about future measurements of electron densities in high redshift clusters constrain the power law index for the magnetic field evolution to better than dm=0.4, if the magnetic field in clusters should follow B ~ (1+z)^m.