The Square Kilometre Array (SKA)

Abstract:

This paper describes a new Heterodyne Array Receiver Program (HARP) and Auto-Correlation Spectral Imaging System (ACSIS) that have recently been installed and commissioned on the James Clerk Maxwell Telescope. The 16-element focal-plane array receiver, operating in the submillimetre from 325 to 375 GHz, offers high (three-dimensional) mapping speeds, along with significant improvements over single-detector counterparts in calibration and image quality. Receiver temperatures are ∼120 K across the whole band, and system temperatures of ∼300 K are reached routinely under good weather conditions. The system includes a single-sideband (SSB) filter so these are SSB values. Used in conjunction with ACSIS, the system can produce large-scale maps rapidly, in one or more frequency settings, at high spatial and spectral resolution. Fully sampled maps of size can be observed in under 1 h. The scientific need for array receivers arises from the requirement for programmes to study samples of objects of statistically significant size, in large-scale unbiased surveys of galactic and extra-galactic regions. Along with morphological information, the new spectral imaging system can be used to study the physical and chemical properties of regions of interest. Its three-dimensional imaging capabilities are critical for research into turbulence and dynamics. In addition, HARPACSIS will provide highly complementary science programmes to wide-field continuum studies and produce the essential preparatory work for submillimetre interferometers such as the Submillimeter Array (SMA) and Atacama Large MillimeterSubmillimeter Array (ALMA). © 2009 RAS.

Abstract:

In this paper we study the relation of radio emission to X-ray spectral and variability properties for a large sample of black hole X-ray binary systems. This is done to test, refine and extend - notably into the timing properties - the previously published 'unified model' for the coupling of accretion and ejection in such sources. In 14 outbursts from 11 different sources we find that in every case the peak radio flux, on occasion directly resolved into discrete relativistic ejections, is associated with the bright hard to soft state transition near the peak of the outburst. We also note the association of the radio flaring with periods of X-ray flaring during this transition in most, but not all, of the systems. In the soft state, radio emission is in nearly all cases either undetectable or optically thin, consistent with the suppression of the core jet in these states and 'relic' radio emission from interactions of previously ejected material and the ambient medium. However, these data cannot rule out an intermittent, optically thin, jet in the soft state. In attempting to associate X-ray timing properties with the ejection events we find a close, but not exact, correspondence between phases of very low integrated X-ray variability and such ejections. In fact the data suggest that there is not a perfect one-to-one correspondence between the radio, X-ray spectral or X-ray timing properties, suggesting that they may be linked simply as symptoms of the underlying state change and not causally to one another. We further study the sparse data on the reactivation of the jet during the transition back to the hard state in decay phase of outbursts, and find marginal evidence for this in one case only. In summary we find no strong evidence against the originally proposed model, confirming and extending some aspects of it with a much larger sample, but note that several aspects remain poorly tested. © 2009 RAS.

Abstract:

We present the results of simultaneous X-ray and radio observations of the peculiar Z-type neutron star X-ray binary Cir X-1, observed with the Rossi X-ray Timing Explorer satellite and the Australia Telescope Compact Array in 2000 October and 2002 December. We identify typical Z-source behaviour in the power density spectra as well as characteristic Z patterns drawn in an X-ray hardness-intensity diagram. Power spectra typical of bright atoll sources have also been identified at orbital phases after the periastron passage, while orbital phases before the periastron passage are characterized by power spectra that are typical neither of Z nor of atoll sources. We investigate the coupling between the X-ray and the radio properties, focusing on three orbital phases when an enhancement of the radio flux density has been detected, to test the link between the inflow (X-ray) and the outflow (radio jet) to/from the compact object. In two out of three cases, we associate the presence of the radio jet to a spectral transition in the X-rays, although the transition does not precede the radio flare, as detected in other Z sources. An analogous behaviour has recently been found in the black hole candidate GX 339-4. In the third case, the radio light curve shows a similar shape to the X-ray light curve. We discuss our results in the context of jet models, considering also black hole candidates. © 2009 RAS.

Abstract:

The S3-Tools are a set of Python-based routines and interfaces whose purpose is to provide user-friendly access to the SKA Simulated Skies (S3) set of simulations, an effort led by the University of Oxford in the framework of the European Union's SKADS program (http://www.skads-eu.org). The databases built from the S3 simulations are hosted by the Oxford e-Research Center (OeRC), and can be accessed through a web portal at http://s-cubed.physics.ox.ac.uk. This paper focuses on the practical steps involved to make radio images from the S3-SEX and S3-SAX simulations using the S 3-Map tool and should be taken as a broad overview. For a more complete description, the interested reader should look up the user's guide. The output images can then be used as input to instrument simulators, e.g. to assess technical designs and observational strategies for the SKA and SKA pathfinders.

Abstract:

Digital beamforming for the aperture array components of the SKA poses considerable computational challenges. In this paper, we propose a hierarchical scheme aimed at tackling them and introduce OSKAR, a beamforming simulator which implements these ideas and algorithms.