7,515 research outputs found
Extremely Anisotropic Scintillations
A small number of quasars exhibit interstellar scintillation on time-scales
less than an hour; their scintillation patterns are all known to be
anisotropic. Here we consider a totally anisotropic model in which the
scintillation pattern is effectively one-dimensional. For the persistent rapid
scintillators J1819+3845 and PKS1257-326 we show that this model offers a good
description of the two-station time-delay measurements and the annual cycle in
the scintillation time-scale. Generalising the model to finite anisotropy
yields a better match to the data but the improvement is not significant and
the two additional parameters which are required to describe this model are not
justified by the existing data. The extreme anisotropy we infer for the
scintillation patterns must be attributed to the scattering medium rather than
a highly elongated source. For J1819+3845 the totally anisotropic model
predicts that the particular radio flux variations seen between mid July and
late August should repeat between late August and mid November, and then again
between mid November and late December as the Earth twice changes its direction
of motion across the scintillation pattern. If this effect can be observed then
the minor-axis velocity component of the screen and the orientation of that
axis can both be precisely determined. In reality the axis ratio is finite,
albeit large, and spatial decorrelation of the flux pattern along the major
axis may be observable via differences in the pairwise fluxes within this
overlap region; in this case we can also constrain both the major-axis velocity
component of the screen and the magnitude of the anisotropy.Comment: 5 pages, 4 figures, MNRAS submitte
Self-organization with equilibration: a model for the intermediate phase in rigidity percolation
Recent experimental results for covalent glasses suggest the existence of an
intermediate phase attributed to the self-organization of the glass network
resulting from the tendency to minimize its internal stress. However, the exact
nature of this experimentally measured phase remains unclear. We modify a
previously proposed model of self-organization by generating a uniform sampling
of stress-free networks. In our model, studied on a diluted triangular lattice,
an unusual intermediate phase appears, in which both rigid and floppy networks
have a chance to occur, a result also observed in a related model on a Bethe
lattice by Barre et al. [Phys. Rev. Lett. 94, 208701 (2005)]. Our results for
the bond-configurational entropy of self-organized networks, which turns out to
be only about 2% lower than that of random networks, suggest that a
self-organized intermediate phase could be common in systems near the rigidity
percolation threshold.Comment: 9 pages, 6 figure
Self-organized criticality in the intermediate phase of rigidity percolation
Experimental results for covalent glasses have highlighted the existence of a
new self-organized phase due to the tendency of glass networks to minimize
internal stress. Recently, we have shown that an equilibrated self-organized
two-dimensional lattice-based model also possesses an intermediate phase in
which a percolating rigid cluster exists with a probability between zero and
one, depending on the average coordination of the network. In this paper, we
study the properties of this intermediate phase in more detail. We find that
microscopic perturbations, such as the addition or removal of a single bond,
can affect the rigidity of macroscopic regions of the network, in particular,
creating or destroying percolation. This, together with a power-law
distribution of rigid cluster sizes, suggests that the system is maintained in
a critical state on the rigid/floppy boundary throughout the intermediate
phase, a behavior similar to self-organized criticality, but, remarkably, in a
thermodynamically equilibrated state. The distinction between percolating and
non-percolating networks appears physically meaningless, even though the
percolating cluster, when it exists, takes up a finite fraction of the network.
We point out both similarities and differences between the intermediate phase
and the critical point of ordinary percolation models without
self-organization. Our results are consistent with an interpretation of recent
experiments on the pressure dependence of Raman frequencies in chalcogenide
glasses in terms of network homogeneity.Comment: 20 pages, 18 figure
Pentagonal puckering in a sheet of amorphous graphene
Ordered graphene has been extensively studied. In this paper we undertake a
first density functional study of it topologically disordered analogues of
graphene, in the form of a random network, consisting predominantly of
hexagonal rings, but also including pentagons and heptagons. After some
preliminaries with crystalline material, we relax various random network models
and find that the presence of carbon pentagons induce local curvature, thus
breaking the initial planar symmetry, in some analogy with the case of
fullerenes. Using density functional theory to calculate the total energy, we
find that while the planar state is locally stable, there is a puckered state
that has lower energy. The scale of the puckering is consistent with that
expected with local maxima and minima associated with pentagons surrounded by
larger rings; forming local "buckyball domes"
Preliminary ultraviolet reflectance of some rocks and minerals from 2000 angstrom to 3000 angstrom
Ultraviolet reflectance measurements of rocks and minerals from 2000 A to 3000
Relativistic and slowing down: the flow in the hotspots of powerful radio galaxies and quasars
Pairs of radio emitting jets with lengths up to several hundred kiloparsecs
emanate from the central region (the `core') of radio loud active galaxies.
In the most powerful of them, these jets terminate in the `hotspots', compact
high brightness regions, where the jet flow collides with the intergalactic
medium (IGM). Although it has long been established that in their inner
(parsec) regions these jet flows are relativistic, it is still not clear
if they remain so at their largest (hundreds of kiloparsec) scales. We argue
that the X-ray, optical and radio data of the hotspots, despite their
at-first-sight disparate properties, can be unified in a scheme involving a
relativistic flow upstream of the hotspot that decelerates to the
sub-relativistic speed of its inferred advance through the IGM and viewed at
different angles to its direction of motion. This scheme, besides providing an
account of the hotspot spectral properties with jet orientation, it also
suggests that the large-scale jets remain relativistic all the way to the
hotspots.Comment: to appear in ApJ
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End users\u27 perceptions concerning computer applications implemented in broadcast stations
In an era of limited resources, organizations must scrutinize every purchase carefully. A critical and sometimes very expensive purchase is for computer hardware and software. The reasons for the purchase are generally given as productivity improvements or personnel reductions. However, assessment of why computer applications are really implemented from the end users\u27 perspective has not been reported. In order to answer questions to justify expenditures it would be helpful to understand how end users perceive the reasons for implementation of computer applications in their stations. To this end a questionnaire was sent to all broadcast radio and TV stations in the State of Wisconsin. The end users (station managers) were requested to identify reasons for present computer application implementations and areas of implementations for future use. The results of this study compare type of computer being used and station characteristics with a checklist of reasons for computer application implementations in the broadcast industry. This research begins the identification of other factors that are Justifiable reasons for computer application implementations. In doing this, end users can broaden the how, why, what, and when of their computer application purchases. We all believe that the computer is a tool to help us do our jobs better, but what does that mean and how can we document these feelings? Or as Jon Van (I99I) and G. P. Zachary (1991) have stated, are we really getting what we expected from the computer applications we have implemented? This research study does not pretend to completely answer these or other questions in this area. However, it is a beginning and valuable research that needs to be augmented
Stressed backbone and elasticity of random central-force systems
We use a new algorithm to find the stress-carrying backbone of ``generic''
site-diluted triangular lattices of up to 10^6 sites. Generic lattices can be
made by randomly displacing the sites of a regular lattice. The percolation
threshold is Pc=0.6975 +/- 0.0003, the correlation length exponent \nu =1.16
+/- 0.03 and the fractal dimension of the backbone Db=1.78 +/- 0.02. The number
of ``critical bonds'' (if you remove them rigidity is lost) on the backbone
scales as L^{x}, with x=0.85 +/- 0.05. The Young's modulus is also calculated.Comment: 5 pages, 5 figures, uses epsfi
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