Alien Registration- Rickett, Alberta M. (Portland, Cumberland County)

https://digitalmaine.com/alien_docs/21590/thumbnail.jp

Alien Registration- Rickett, Alberta M. (Portland, Cumberland County)

https://digitalmaine.com/alien_docs/21590/thumbnail.jp

Electric field representation of pulsar intensity spectra

Pulsar dynamic spectra exhibit high visibility fringes arising from interference between scattered radio waves. These fringes may be random or highly ordered patterns, depending on the nature of the scattering or refraction. Here we consider the possibility of decomposing pulsar dynamic spectra -- which are intensity measurements -- into their constituent scattered waves, i.e. electric field components. We describe an iterative method of achieving this decomposition and show how the algorithm performs on data from the pulsar B0834+06. The match between model and observations is good, although not formally acceptable as a representation of the data. Scattered wave components derived in this way are immediately useful for qualitative insights into the scattering geometry. With some further development this approach can be put to a variety of uses, including: imaging the scattering and refracting structures in the interstellar medium; interstellar interferometric imaging of pulsars at very high angular resolution; and mitigating pulse arrival time fluctuations due to interstellar scattering.Comment: 7 Pages, 2 Figures, revised version, accepted by MNRA

Radio Scintillation due to Discontinuities in the Interstellar Plasma Density

We develop the theory of interstellar scintillation as caused by an irregular plasma having a power-law spatial density spectrum with a spectral exponent of 4 corresponding to a medium with abrupt changes in its density. An ``outer scale'' is included in the model representing the typical scale over which the density of the medium remains uniform. Such a spectrum could be used to model plasma shock fronts in supernova remnants or other plasma discontinuities. We investigate and develop equations for the decorrelation bandwidth of diffractive scintillations and the refractive scintillation index and compare our results with pulsar measurements. We consider both a medium concentrated in a thin layer and an extended irregular medium. We conclude that the discontinuity model gives satisfactory agreement for many diffractive measurements, in particular the VLBI meaurements of the structure function exponent between 5/3 and 2. However, it gives less satisfactory agreement for the refractive scintillation index than does the Kolmogorov turbulence spectrum. The comparison suggests that the medium consists of a pervasive background distribution of turbulence embedded with randomly placed discrete plasma structures such as shocks or HII regions. This can be modeled by a composite spectrum following the Kolmogorov form at high wavenumbers and steepening at lower wavenumbers corresponding to the typical (inverse) size of the discrete structures. Such a model can also explain the extreme scattering events. However, lines of sight through the enhanced scattering prevalent at low galactic latitudes are accurately described by the Kolmogorov spectrum in an extended medium and do not appear to have a similar low-wavenumber steepening.Comment: Accpeted for ApJ vol 531, March 200

What are the best prophylactic drugs for migraine?

Beta-blockers without intrinsic sympathomimetic activity, amitriptyline, divalproex sodium/sodium valproate, and topiramate are the most effective drugs for preventing episodic migraine (strength of recommendation: A, multiple, well-designed, randomized controlled trials [RCTs])

Modeling of Interstellar Scintillation Arcs from Pulsar B1133+16

The parabolic arc phenomenon visible in the Fourier analysis of the scintillation spectra of pulsars provides a new method of investigating the small scale structure in the ionized interstellar medium (ISM). We report archival observations of the pulsar B1133+16 showing both forward and reverse parabolic arcs sampled over 14 months. These features can be understood as the mutual interference between an assembly of discrete features in the scattered brightness distribution. By model-fitting to the observed arcs at one epoch we obtain a ``snap-shot'' estimate of the scattered brightness, which we show to be highly anisotropic (axial ratio >10:1), to be centered significantly off axis and to have a small number of discrete maxima, which are coarser the speckle expected from a Kolmogorov spectrum of interstellar plasma density. The results suggest the effects of highly localized discrete scattering regions which subtend 0.1-1 mas, but can scatter (or refract) the radiation by angles that are five or more times larger.Comment: 14 pages, 4 figures, submitted to Astrophysical Journa

Interstellar Scintillation of the Polarized Flux Density in Quasar, PKS 0405-385

The remarkable rapid variations in radio flux density and polarization of the quasar PKS 0405-385 observed in 1996 are subject to a correlation analysis, from which characteristic time scales and amplitudes are derived. The variations are interpreted as interstellar scintillations. The cm wavelength observations are in the weak scintillation regime for which models for the various auto- and cross-correlations of the Stokes parameters are derived and fitted to the observations. These are well modelled by interstellar scintillation (ISS) of a 30 by 22 micro-as source, with about 180 degree rotation of the polarization angle along its long dimension. This success in explaining the remarkable intra-day variations (IDV)in polarization confirms that ISS gives rise to the IDV in this quasar. However, the fit requires the scintillations to be occurring much closer to the Earth than expected according to the standard model for the ionized interstellar medium (IISM). Scattering at distances in the range 3-30 parsec are required to explain the observations. The associated source model has a peak brightness temperature near 2.0 10^{13}K, which is about twenty-five times smaller than previously derived for this source. This reduces the implied Doppler factor in the relativistic jet, presumed responsible to 10-20, high but just compatible with cm wavelength VLBI estimates for the Doppler factors in Active Galactic Nuclei (AGNs).Comment: 43 pages 15 figures, accepted for ApJ Dec 200

Simulation of the interstellar scintillation and the extreme scattering events of pulsars

The rare and conspicuous flux density variations of some radio sources (extragalactic and pulsars) for periods of weeks to months have been denoted Extreme Scattering Events (ESE's) by Fiedler et al. (1987). Presently, there is no astrophysical mechanism that satisfactorily produces this phenomenon. In this paper, we conjecture that inhomogeneities of the electronic density in the turbulent interstellar medium might be the origin of this phenomenon. We have tested this conjecture by a simulation of the scintillation of the pulsar B1937+21 at 1.4 GHz and 1.7 GHz for a period of six months. To this end, we have constructed a large square Kolmogorov phase screen made of 131k x 131k pixels with electron inhomogeneity scales ranging from 6 x $10^6$ m to $10^{12}$ m and used the Kirchhoff-Fresnel integral to simulate dynamic spectra of a pulsar within the framework of Physical Optics. The simulated light curves exhibit a 10 day long variation simultaneously at 1.41 and 1.7 GHz that is alike the ``ESE'' observed with the Nancay radiotelescope toward the pulsar B1937+21 in October 1989. Consequently, we conclude that ``ESE'' toward pulsars can be caused naturally by the turbulence in the ionized interstellar medium instead of invoking the crossing of discrete over pressured ionized clouds on the line of sight as in the model of Fiedler et al. (1987). We suggest that longer events could occur in a simulation of scintillation, if larger electron inhomogeneities > $10^{12}$ m were included in the construction of the Kolmogorov phase screen. This next step requires a supercomputer.Comment: Accepted for publication in Astronomy & Astrophysic

Ageing, drama and creativity: translating research into practice

Ageing, Drama, and Creativity was a pilot six-session interprofessional training course delivered collaboratively by Keele University and the New Vic Theatre, Newcastle-under-Lyme, as part of our Arts and Humanities Research Council (AHRC) funded Ages and Stages follow-on project. The course brought together a critical gerontological approach with arts-based educational practices and was designed to develop practice capabilities and age awareness among a diverse group of professionals working in arts organizations, the voluntary sector, local government, health and social services, and housing. This article describes how the course was developed and how participants were selected, details its aims and objectives, provides an overview of the sessions and a flavor of some of the exercises that were used, and considers findings from the structured evaluation alongside written reflections from participants