The Square Kilometre Array (SKA)

Abstract:

Recent studies of different X-ray binaries (XRBs) have shown a clear correlation between the radio and X-ray emission. We present evidence of a close relationship found between the radio and X-ray emission at different epochs for GRS 1915+105, using observations from the Ryle Telescope and Rossi X-ray Timing Explorer satellite. The strongest correlation was found during the hard state (also known as the "plateau" state), where a steady AU-scale jet is known to exist. Both the radio and X-ray emission were found to decay from the start of most plateau states, with the radio emission decaying faster. An empirical relationship of was then fitted to data taken only during the plateau state, resulting in a power-law index of ξ ~ 1.7 ± 0.3, which is significantly higher than in other black hole XRBs in a similar state. An advection-flow model was then fitted to this relationship and compared to the universal XRB relationship as described by Gallo et al. (2003, MNRAS, 344, 60). We conclude that either (I) the accretion disk in this source is radiatively efficient, even during the continuous outflow of a compact jet, which could also suggest a universal turn-over from radiatively inefficient to efficient for all stellar-mass black holes at a critical mass accretion rate (M c≈1018.5 g/s); or (II) the X-rays in the plateau state are dominated by emission from the base of the jet and not the accretion disk (e.g. via inverse Compton scattering from the outflow). © 2010 ESO.

Abstract:

We have detected an Hα bow shock nebula around PSR J1741-2054, a pulsar discovered through its GeV γ-ray pulsations. The pulsar is only ∼1'.5 behind the leading edge of the shock. Optical spectroscopy shows that the nebula is non-radiative, dominated by Balmer emission. The Hα images and spectra suggest that the pulsar wind momentum is equatorially concentrated and implies a pulsar space velocity ≈150kms-1, directed 15° ± 10° out of the plane of the sky. The complex Hα profile indicates that different portions of the post-shock flow dominate line emission as gas moves along the nebula and provide an opportunity to study the structure of this unusual slow non-radiative shock under a variety of conditions. CXO ACIS observations reveal an X-ray pulsar wind nebula within this nebula, with a compact ∼2.5 equatorial structure and a trail extending several arcminutes behind. Together these data support a close (≤0.5 kpc) distance, a spin geometry viewed edge-on, and highly efficient γ-ray production for this unusual, energetic pulsar. © 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

Abstract:

In this paper, we consider the problem of optimum multi-domain real-time beamforming and high-precision beam pattern positioning in application to very large wideband array antennas, particularly to the Square Kilometre Array (SKA) aperture array antenna. We present a new structure for wideband space-frequency beamforming and beamsteering that maximizes detectability of cosmic signals over the array operational frequency range. © 2010 IEEE.

Abstract:

Gigahertz-frequency phased arrays that are integral to next-generation radio astronomy instruments, in particular the Square Kilometre Array (SKA) radio telescope, pose a significant signal processing challenge. We argue that the development of high-performance signal processing systems is critical, not only for the particular application to future radio astronomy instrumentation, but also to the entire field of ultra-wideband phased arrays in the gigahertz bandwidth range. To this end, we have developed a cycle-accurate simulator environment and programming language for a novel, massively multicore array processor, and prototyped on it representative digital frequency domain algorithms. The results of this analysis reveal beamforming as a low compute-to-I/O processing task, best suited to high- I/O-bandwidth, streaming signal processing systems. © 2010 IEEE.

Abstract:

Digital beamforming for the aperture array components of the Square Kilometre Array (SKA) poses considerable computational challenges. We propose a hierarchical scheme aimed at tackling them and introduce OSKAR, a beamforming simulator which implements these ideas and algorithms. The simulator continues to be developed to investigate possible designs for the custom devices envisaged for phase 1 of the SKA construction. © 2010 IEEE.