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We consider the stability of nonlinear travelling waves in a class of activator-inhibitor systems. The eigenvalue equation arising from linearizing about the wave is seen to preserve the manifold of Lagrangian planes for a nonstandard symplectic form. This allows us to define a Maslov index for the wave corresponding to the spatial evolution of the unstable bundle. We formulate the Evans function for the eigenvalue problem and show that the parity of the Maslov index determines the sign of the derivative of the Evans function at the origin. The connection between the Evans function and the Maslov index is established by a ‘detection form,’ which identifies conjugate points for the curve of Lagrangian planes.
We describe the performance of the Boolardy Engineering Test Array, the prototype for the Australian Square Kilometre Array Pathfinder telescope. Boolardy Engineering Test Array is the first aperture synthesis radio telescope to use phased array feed technology, giving it the ability to electronically form up to nine dual-polarisation beams. We report the methods developed for forming and measuring the beams, and the adaptations that have been made to the traditional calibration and imaging procedures in order to allow BETA to function as a multi-beam aperture synthesis telescope. We describe the commissioning of the instrument and present details of Boolardy Engineering Test Array’s performance: sensitivity, beam characteristics, polarimetric properties, and image quality. We summarise the astronomical science that it has produced and draw lessons from operating Boolardy Engineering Test Array that will be relevant to the commissioning and operation of the final Australian Square Kilometre Array Path telescope.
This paper describes the system architecture of a newly constructed radio telescope – the Boolardy engineering test array, which is a prototype of the Australian square kilometre array pathfinder telescope. Phased array feed technology is used to form multiple simultaneous beams per antenna, providing astronomers with unprecedented survey speed. The test array described here is a six-antenna interferometer, fitted with prototype signal processing hardware capable of forming at least nine dual-polarisation beams simultaneously, allowing several square degrees to be imaged in a single pointed observation. The main purpose of the test array is to develop beamforming and wide-field calibration methods for use with the full telescope, but it will also be capable of limited early science demonstrations.
The Parkes pulsar data archive currently provides access to 144044 data files obtained from observations carried out at the Parkes observatory since the year 1991. Around 105 files are from surveys of the sky, the remainder are observations of 775 individual pulsars and their corresponding calibration signals. Survey observations are included from the Parkes 70 cm and the Swinburne Intermediate Latitude surveys. Individual pulsar observations are included from young pulsar timing projects, the Parkes Pulsar Timing Array and from the PULSE@Parkes outreach program. The data files and access methods are compatible with Virtual Observatory protocols. This paper describes the data currently stored in the archive and presents ways in which these data can be searched and downloaded.
This study presents the first analysis of the impact of NASCAR
sponsorship announcements on the stock prices of sponsoring firms. The
primary finding of the study—that NASCAR sponsorship
announcements were accompanied by the largest increases in shareholder
wealth ever recorded in the marketing literature in response to a
voluntary marketing program—represents a striking and unambiguous
stock market endorsement of the sponsorships. Indeed, the 24 sponsors
analyzed in this study experienced mean increases in shareholder wealth
of over $300 million dollars, net of all of the costs associated with
the sponsorships. A multiple regression analysis of firm-specific stock
price changes and select corporate and sponsorship attributes indicates
that NASCAR sponsorships with more successful racing teams, corporate
(as opposed to product or divisional) sponsorships, and sponsorships
with direct ties to the consumer automotive industry are all positively
correlated with perceived sponsorship success, while corporate cash
flow per share (a well-known proxy for agency conflicts within the
firm) is negatively related with shareholder approval.
We are not convinced by Gangestad & Simpson that differential mating strategies within each sex would be greater than such strategies between sexes. The target article does not provide actual evidence of human males who do not desire mating with multiple females, or evidence that the benefits for females of short-term matings with multiple males have ever outweighed the associated costs.
We discuss the implementation of classical molecular dynamics simulations on Single- Instruction Multiple-Data computers with two-dimensional mesh connectivity. Specific timing results are given for the MasPar MP-1, complemented by a general time complexity analysis. Our main interest is in simulations with a number of particles N that is comparable to the number of processors P. In particular, we discuss results for large Si-clusters with up to 2000 particles, using a Stillinger-Weber potential. Particular attention is paid to the problem of handling in an efficient way the calculation of the three-body potential and force.
Our ideas of how to image objects have progressed by leaps and bounds in the last twenty to thirty years. We now have a sophisticated understanding of many clever and subtle approaches to imaging. There are a few outstanding contributions which have provoked flurries of development and acheivement in many different areas: top of my list would be the principle of aperture synthesis, Jennison's closure phase, Högbom's CLEAN algorithm and Labeyrie's speckles. In addition, we have benefited tremendously from developments in computing hardware, software and algorithms (the most spectacular being the Cooley-Tukey Fast Fourier Transform). Each one of these contributions did not so much spur development in existing areas as open up entirely new vistas of possibilities. For example, Jennison's closure phase is rarely directly used by radio-interferometrists now but it did show, particularly when developed in VLBI, that imaging in the presence of severe phase errors is possible. This success then encouraged the two separate developments. First in the more flexible selfcalibration routines in which closure is implicit and, second, in pushing imaging interferometric arrays to shorter and shorter wavelengths, now ending up in the infra-red and optical regimes. Following on from these great works, many people have made lesser but still vital contributions. The example of speckle comes to mind as one where many people have had a hand in determining what is now standard technique.
We have run a number of simulations investigating the limitations of noiseless point spread deconvolution with the VLA. On-source errors of more than .01 and off-source dynamic ranges of less than 15,000 were found for some parameters typical of high precision observations. Deconvolution errors were not confined to the support of the source, and limit the dynamic range in at least one VLA observation.
We have performed an observational test of the “Homogeneous Array” concept utilized in the MMA design, whereby the array elements are used in total power mode to provide the shortest spacings. Using the VLA in interferometric mode and a VLBA element measuring total power, we have reconstructed a mosaic image of the Crab Nebula, which fits the data adequately and is of good visual appearance.
The P-band is the most recently completed observing band at the VLA. The aperture efficiency of the antennas is 0.4 and the system temperature is ∼ 130 K. The primary beam is slightly off-axis and its half-power width is steeply frequency dependent, which induces abnormal spectral indices on off-axis (≥ 1°) sources. We have searched for Zel’dovich “pancakes” and reached a level of ΔSrms ∼ 3.5 mJy for spectral channels of width Δν ∼ 100 kHz. We find no pancake with MHI ≳5 × 1014 M⊙.
The general relation between interferometric data and sky brightness requires the use of three-dimensional Fourier transforms. Only in special cases does the standard two-dimensional Fourier relations result in images free of distortions. For the VLA, use of two-dimensional transforms introduces errors which are important at low frequencies and high resolutions. We review options and progress in solving the computational problems associated with the full solution.
We have obtained radio continuum observations of an HH object, known as the “streamer” in Orion using the VLA in a number of configurations at λ2, 6 and 20cm. This object is located several arc-minutes to the north of HH34 in L1641 cloud and exhibits a remarkably long and narrow structure with dimensions of 5′ × 10′ in optical photographs unlike the morphology of any other HH object. Based on radio continuum observations, we have detected a significant linearly polarized emission at λ6cm from the core of the streamer indicating highly uniform magnetic field geometry. This unusual result suggests that the source responsible for producing the nebulosity is a nonthermal-emitting radio source. Because of the presence of a number of newly born low-mass stars in L1641 and because of the detection of a highly reddened 2μm source coincident with the synchrotron source, it is likely that the source at the core of the streamer is a low-mass star responsible for the non-thermal emission and the HH nebulosity. Presence of synchrotron emission associated with a newly born star supports further view that the magnetic field plays a strong role in the early phase of star formation.
The Nuclear Radio cores of several nearby extended radio galaxies (e.g. M87, 3C236) consist not only of optically thick (< 1 pc) components, but also of emission on somewhat larger scale. As extended radio sources associated with quasars have on average stronger and more luminous radio cores (see e.g. Miley, 1980, Ann. Rev. Astron. Astrophys. 18, 165), we have started a project to study the properties of these quasar cores.
Bridle (this volume) has summarized the overall characteristics of the jets found in numerous low-luminosity and some high-luminosity radio sources. Previous observations made with the partially completed VLA at wavelengths of 6 and 20 cm indicated that 3C449 was an archetypal radio source obeying all the “rules” summarized by Bridle. New observations with the VLA of the polarization structure at 6 and 20 cm have destroyed this simple picture and identify 3C449 as a “rogue” jet source.
The problem of estimating radio sky brightness distributions from incomplete, and noisy, visibility measurements, such as those collected by a long baseline interferometer, has recently been attacked using nonlinear data-adaptive techniques such as the Maximum Entropy method (Abies (1974), Wernecke (1976), Wernecke and D’Addario (1977), Gull and Daniell (1978)) and the Maximum Likelihood method (Papadopoulos (1975)).
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