399 research outputs found
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
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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 m to
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 > 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
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