37,524 research outputs found
The main- interpulse interaction of PSR B1702-19
This paper reports on single-pulse radio observations of PSR B1702-19 and their implications for pulsar emission theories. These observations were made with the Westerbork Synthesis Radio Telescope at 1380 and 328 MHz. The PA-swing is used to constrain possible geometries of the pulsar and the single-pulse data is analysed for subpulse modulation correlations between the main pulse and interpulse. We confirm earlier conclusions that the dipole axis of this pulsar is almost perpendicular to its rotation axis, and report that both its main pulse and interpulse are modulated with a periodicity around 10.4 times the pulsar's rotation. Allowing for the half-period delay between main pulse and interpulse the modulation is found to be precisely in phase. Despite small secular variations in the periodicity, the phase-locking continues over all timescales ranging up to several years. The precision of the phase locking is difficult for current emission theories to explain if the main pulse and interpulse originate from opposing magnetic poles. We therefore also explore the possibility of a bidirectional model, in which all the modulated emission comes from one pole, but is seen from two sides and slightly displaced by aberration and time-delay. In this model the unmodulated emission is directed to us from the opposite pole, requiring the emission of the main pulse to originate from two different poles. This is difficult to reconcile with the observed smooth PA-swing. Whichever model turns out to be correct, the answer will have important implications for emission theories
Thermal barrier coatings issues in advanced land-based gas turbines
The Department of Energy's Advanced Turbine System (ATS) program is aimed at forecasting the development of a new generation of land-based gas turbine systems with overall efficiencies significantly beyond those of current state-of-the-art machines, as well as greatly increased times between inspection and refurbishment, improved environmental impact, and decreased cost. The proposed duty cycle of ATS turbines will require the use of different criteria in the design of the materials for the critical hot gas path components. In particular, thermal barrier coatings will be an essential feature of the hot gas path components in these machines. While such coatings are routinely used in high-performance aircraft engines and are becoming established in land-based turbines, the requirements of the ATS turbine application are sufficiently different that significant improvements in thermal barrier coating technology will be necessary. In particular, it appears that thermal barrier coatings will have to function on all airfoil sections of the first stage vanes and blades to provide the significant temperature reduction required. In contrast, such coatings applied to the blades and vances of advanced aircraft engines are intended primarily to reduce air cooling requirements and extend component lifetime; failure of those coatings can be tolerated without jeopardizing mechanical or corrosion performance. A major difference is that in ATS turbines these components will be totally reliant on thermal barrier coatings which will, therefore, need to be highly reliable even over the leading edges of first stage blades. Obviously, the ATS program provides a very challenging opportunity for TBC's, and involves some significant opportunities to extend this technology
Is pulsar B0656+14 a very nearby RRAT source?
The recently discovered RRAT sources are characterized by very bright radio
bursts which, while being periodically related, occur infrequently. We find
bursts with the same characteristics for the known pulsar B0656+14. These
bursts represent pulses from the bright end of an extended smooth pulse-energy
distribution and are shown to be unlike giant pulses, giant micropulses or the
pulses of normal pulsars. The extreme peak-fluxes of the brightest of these
pulses indicates that PSR B0656+14, were it not so near, could only have been
discovered as an RRAT source. Longer observations of the RRATs may reveal that
they, like PSR B0656+14, emit weaker emission in addition to the bursts.Comment: 4 pages, 4 figures, accepted by ApJ
What can polysemy tell us about theories of explanation?
Philosophical accounts of scientific explanation are broadly divided into ontic and epistemic views. This paper explores the idea that the lexical ambiguity of the verb to explain and its nominalisation supports an ontic conception of explanation. I analyse one argument which challenges this strategy by criticising the claim that explanatory talk is lexically ambiguous, 375ā394, 2012). I propose that the linguistic mechanism of transfer of meaning, 109ā132, 1995) provides a better account of the lexical alternations that figure in the systematic polysemy of explanatory talk, and evaluate the implications of this proposal for the debate between ontic and epistemic conceptions of scientific explanation
Ornstein-Zernike equation and Percus-Yevick theory for molecular crystals
We derive the Ornstein-Zernike equation for molecular crystals of axially
symmetric particles and apply the Percus-Yevick approximation to this system.
The one-particle orientational distribution function has a nontrivial
dependence on the orientation and is needed as an input. Despite some
differences, the Ornstein-Zernike equation for molecular crystals has a similar
structure as for liquids. We solve both equations for hard ellipsoids on a sc
lattice. Compared to molecular liquids, the tensorial orientational correlators
exhibit less structure. However, depending on the lengths a and b of the
rotation axis and the perpendicular axes of the ellipsoids, different behavior
is found. For oblate and prolate ellipsoids with b >= 0.35 (units of the
lattice constant), damped oscillations in distinct directions of direct space
occur for some correlators. They manifest themselves in some correlators in
reciprocal space as a maximum at the Brillouin zone edge, accompanied by maxima
at the zone center for other correlators. The oscillations indicate alternating
orientational fluctuations, while the maxima at the zone center originate from
nematic-like orientational fluctuations. For a <= 2.5 and b <= 0.35, the
oscillations are weaker. For a >= 3.0 and b <= 0.35, no oscillations occur any
longer. For many of the correlators in reciprocal space, an increase of a at
fixed b leads to a divergence at the zone center q = 0, consistent with
nematic-like long range fluctuations, and for some oblate and prolate systems
with b ~< 1.0 a simultaneous tendency to divergence of few other correlators at
the zone edge is observed. Comparison with correlators from MC simulations
shows satisfactory agreement. We also obtain a phase boundary for
order-disorder transitions.Comment: 20 pages, 13 figures, submitted to Phys. Rev.
Phase fluctuations in anisotropic Bose condensates: from cigars to rings
We study the phase-fluctuating condensate regime of ultra-cold atoms trapped
in a ring-shaped trap geometry, which has been realized in recent experiments.
We first consider a simplified box geometry, in which we identify the
conditions to create a state that is dominated by thermal phase-fluctuations,
and then explore the experimental ring geometry. In both cases we demonstrate
that the requirement for strong phase fluctuations can be expressed in terms of
the total number of atoms and the geometric length scales of the trap only. For
the ring-shaped trap we discuss the zero temperature limit in which a
condensate is realized where the phase is fluctuating due to interactions and
quantum fluctuations. We also address possible ways of detecting the phase
fluctuating regime in ring condensates.Comment: 10 pages, 5 figures, minor edit
Ill-posedness of degenerate dispersive equations
In this article we provide numerical and analytical evidence that some
degenerate dispersive partial differential equations are ill-posed.
Specifically we study the K(2,2) equation and
the "degenerate Airy" equation . For K(2,2) our results are
computational in nature: we conduct a series of numerical simulations which
demonstrate that data which is very small in can be of unit size at a
fixed time which is independent of the data's size. For the degenerate Airy
equation, our results are fully rigorous: we prove the existence of a compactly
supported self-similar solution which, when combined with certain scaling
invariances, implies ill-posedness (also in )
Fit for work? Health, employability and challenges for the UK welfare reform agenda
This article introduces a special issue of Policy Studies entitled āFit for work? Health, employability and challenges for the UK welfare reform agendaā. Growing from a shared concern over the need to expand the evidence base around the processes that led to large numbers of people claiming disability benefits in the UK, it brings together contributions from leading labour market and social policy researchers providing evidence and commentary on major reforms to Incapacity Benefit (IB) in the UK. This special issue address three key questions: what are the main causes of the long-term rise in the number of people claiming IBs; what will reduce the number of claimants; and what is likely to deliver policy effectively and efficiently? This introduction first explains and examines the challenges to reforms to IB in the UK, and then, in conclusion, highlights the answers to the previous three questions ā first, labour market restructuring and marginalisation have driven the rise in numbers claiming IBs. Second, economic regeneration in the Britainās less prosperous areas coupled with intensive and sustained supply-side support measures will bring numbers down. Third, delivery need to be flexible and tailored to individual needs and needs to be able to access local and expert knowledge in a range of organisations, including Job Centre Plus, the NHS as well as the private and voluntary sectors
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