The Square Kilometre Array (SKA)

Abstract:

We describe the development of two circularly symmetric antennas with high polarization purity and low spillover. Both were designed to be used in an all-sky polarization and intensity survey at 5 GHz (the C-Band All-Sky Survey, C-BASS). The survey requirements call for very low levels of cross-polar leakage and far-out sidelobes. Two different existing antennas, with 6.1-m and 7.6-m diameter primaries, were adapted by replacing the feed and secondary optics, resulting in identical beam performances of 0.73deg FWHM, cross-polarization better than -50 dB, and far-out sidelobes below -70 dB. The polarization purity was realized by using a symmetric low-loss dielectric foam support structure for the secondary mirror, avoiding the need for secondary support struts. Ground spill-over was largely reduced by using absorbing baffles around the primary and secondary mirrors, and by the use of a low-sidelobe profiled corrugated feedhorn. The 6.1-m antenna and receiver have been completed and test results show that the optics meet their design goals.

Abstract:

The Q/U Imaging ExperimenT (QUIET) employs coherent receivers at 43GHz and 94GHz, operating on the Chajnantor plateau in the Atacama Desert in Chile, to measure the anisotropy in the polarization of the cosmic microwave background (CMB). QUIET primarily targets the B modes from primordial gravitational waves. The combination of these frequencies gives sensitivity to foreground contributions from diffuse Galactic synchrotron radiation. Between 2008 October and 2010 December, over 10,000hr of data were collected, first with the 19 element 43 GHz array (3458hr) and then with the 90 element 94 GHz array. Each array observes the same four fields, selected for low foregrounds, together covering ≈1000 deg2. This paper reports initial results from the 43 GHz receiver, which has an array sensitivity to CMB fluctuations of 69μK√s. The data were extensively studied with a large suite of null tests before the power spectra, determined with two independent pipelines, were examined. Analysis choices, including data selection, were modified until the null tests passed. Cross-correlating maps with different telescope pointings is used to eliminate a bias. This paper reports the EE, BB, and EB power spectra in the multipole range ℓ = 25-475. With the exception of the lowest multipole bin for one of the fields, where a polarized foreground, consistent with Galactic synchrotron radiation, is detected with 3σ significance, the E-mode spectrum is consistent with the ΛCDM model, confirming the only previous detection of the first acoustic peak. The B-mode spectrum is consistent with zero, leading to a measurement of the tensor-to-scalar ratio of r = 0.35+1.06-0.87. The combination of a new time-stream "double-demodulation" technique, side-fed Dragonian optics, natural sky rotation, and frequent boresight rotation leads to the lowest level of systematic contamination in the B-mode power so far reported, below the level of r = 0.1. © 2011. The American Astronomical Society. All rights reserved.

Abstract:

Jets have been detected in many accreting compact objects, and recently indications for jets have finally been found for transient cataclysmic variables (dwarf novae). However, so far, there have been no convincing reports of radio emission from white dwarfs undergoing stable disc accretion at a high rate, the so-called nova-like variables. Here, we present the first reproducible radio detection of a nova-like cataclysmic variable. The accretion rate and the distance of the detected source V3885 Sgr are comparable to the dwarf nova SS Cyg during its plateau phase. The detected radio emission is also of a similar level, although the source seems to show a steep spectrum. Besides V3885 Sgr, we have also observed IX Vel as well as reanalysed the available data for AC Cnc. Due to dynamic range limitations for IX Vel, we were not able to reach the required sensitivity and only obtained an upper limit. For AC Cnc we cannot confirm the previous detection. We discuss the detection of V3885 Sgr in the context of other types of accreting objects and conclude that the most likely source of the radio emission is optically thin synchrotron emission originating in a jet. Thus, tentative evidence for jets has now been found in both steady and transient CVs, making a universal connection between disc accretion and jet formation possible. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Abstract:

The identification and subsequent discovery of fast radio transients using blind-search surveys require a large amount of processing power, in worst cases scaling as. For this reason, survey data are generally processed off-line, using high-performance computing architectures or hardware-based designs. In recent years, graphics processing units (GPUs) have been extensively used for numerical analysis and scientific simulations, especially after the introduction of new high-level application programming interfaces. Here, we show how GPUs can be used for fast transient discovery in real time. We present a solution to the problem of de-dispersion, providing performance comparisons with a typical computing machine and traditional pulsar processing software. We describe the architecture of a real-time, GPU-based transient search machine. In terms of performance, our GPU solution provides a speed-up factor of between 50 and 200, depending on the parameters of the search. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.