The Square Kilometre Array (SKA)

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.

Abstract:

In response to major changes in the mass accretion rate within the inner accretion flow, black hole binary transients undergo dramatic evolution in their X-ray timing and spectral behaviour during outbursts. In recent years a paradigm has arisen in which 'soft' X-ray states are associated with an inner disc radius at, or very close to, the innermost stable circular orbit (ISCO) around the black hole, while in 'hard' X-ray states the inner edge of the disc is further from the black hole. Models of advective flows suggest that as the X-ray luminosity drops in hard states, the inner disc progressively recedes, from a few gravitational radii (Rg) at the ISCO, to hundreds of Rg. Recent observations which show broad iron line detections and estimates of the disc component strength suggest that a non-recessed disc could still be present in bright hard states. In this study we present a comprehensive analysis of the spectral components associated with the inner disc, utilizing bright states data from X-ray missions with sensitive low-energy responses (e.g. Swift, SAX), including re-analyses of previously published results. A key component of the study is to fully estimate systematic uncertainties associated with such spectral fits. In particular we investigate in detail the effect on the measured disc flux and radius of having a hydrogen column density that is fixed or free to vary. We conclude that at X-ray luminosities above ∼0.01 of the Eddington limit, systematic uncertainties only allow us to constrain the disc to be ≲10Rg from spectral fits. There is, however, clear evidence that at X-ray luminosities between 10-2 and 10-3 of the Eddington rate, the disc does begin to recede. We include measurements of disc radii in two quiescent black hole binaries at bolometric luminosities of <10-7 Eddington, and present the inferred evolution of disc luminosity, temperature, inner radius and accretion rate/efficiency across the entire range of bolometric luminosities 10-8-1 Eddington. We compare our results with theoretical models, and note that the implied rate of disc recession with luminosity is consistent with recent empirical results on the X-ray timing behaviour of black holes of all masses. © 2009 RAS.

Abstract:

We report on Hobby-Eberly Telescope and Palomar 5 m spectroscopy of recently identified γ-ray blazars in the Fermi LAT Bright Source List. These data provide identifications for 10 newly discovered γ-ray flat spectrum radio quasars and six new BL Lacertae (BL Lac) objects plus improved spectroscopy for six additional BL Lac objects. We substantially improve the identification completeness of the bright LAT blazars and give new redshifts and z constraints, new estimates of the black hole masses, and new measurements of the optical spectral energy distribution. © 2009 The American Astronomical Society. All rights reserved.

Abstract:

We have detected significant rotation measure (RM) variations for nine bright pulsars, as a function of pulse longitude. An additional sample of 10 pulsars showed a rather constant RM with phase, yet a small degree of RM fluctuation is visible in at least three of those cases. In all cases, we have found that the rotation of the polarization position angle across our 1.4 GHz observing band is consistent with the λ2 law of interstellar Faraday rotation. We provide for the first time convincing evidence that RM variations across the pulse are largely due to interstellar scattering, although we cannot exclude that magnetospheric Faraday rotation may still have a minor contribution; alternative explanations of this phenomenon, like erroneous de-dispersion and the presence of non-orthogonal polarization modes, are excluded. If the observed, phase-resolved RM variations are common amongst pulsars, then many of the previously measured pulsar RMs may be in error by as much as a few tens of rad m-2. © 2009 RAS.

Abstract:

The medium of RF signal transfer adopted for 2PAD was twisted pair differential signal cabling. While intended to demonstrate a low cost solution, the technique has brought with it several challenges, not least in terms of the losses and phase skew introduced. Some interesting engineering challenges have been faced along the way to delivering usable signals to the DSP Engineers. Faced with an aggressive RFI environment, with strong TV and GSM broadcast signals, a small, reasonably quiet band was exploited. A review of the RFI shielding policy has been required to maintain stability in the gain stages of the analogue system. An effective cabinet RFI barrier has been successfully demonstrated. Future work through PrepSKA will explore alternative cabling solutions, such as coaxial and optical fibre, with the intention of evaluating the main contenders against the cost, power, and performance requirements for SKA.