5,575 research outputs found
Competition, quality and contract compliance: evidence from compulsory competitive tendering in local government in Great Britain, 1987-2000
The introduction of competition has frequently been found to cause costs to fall. There has, however, been a question as to whether this was partly achieved at the cost of quality. Auction theory predicts prices would fall more the greater the competition to provide the service. There has been some debate about whether the smaller budgets would make contract compliance more difficult. Evidence is found in support of this hypothesis. We also find some evidence that the better recorded performance of the in-house direct service organisations (DSOs) during this period was due to the information advantage they had from being incumbents
IC 4406: a radio-infrared view
IC 4406 is a large (about 100'' x 30'') southern bipolar planetary nebula,
composed of two elongated lobes extending from a bright central region, where
there is evidence for the presence of a large torus of gas and dust. We show
new observations of this source performed with IRAC (Spitzer Space Telescope)
and the Australia Telescope Compact Array. The radio maps show that the flux
from the ionized gas is concentrated in the bright central region and
originates in a clumpy structure previously observed in H_alpha, while in the
infrared images filaments and clumps can be seen in the extended nebular
envelope, the central region showing toroidal emission. Modeling of the
infrared emission leads to the conclusion that several dust components are
present in the nebula.Comment: 22 pages, 7 figures, accepted for publication in The Astrophysical
Journal; v.2 has changes in both figures and content; preprint forma
Four-vortex motion around a circular cylinder
The motion of two pairs of counter-rotating point vortices placed in a
uniform flow past a circular cylinder is studied analytically and numerically.
When the dynamics is restricted to the symmetric subspace---a case that can be
realized experimentally by placing a splitter plate in the center plane---, it
is found that there is a family of linearly stable equilibria for same-signed
vortex pairs. The nonlinear dynamics in the symmetric subspace is investigated
and several types of orbits are presented. The analysis reported here provides
new insights and reveals novel features of this four-vortex system, such as the
fact that there is no equilibrium for two pairs of vortices of opposite signs
on the opposite sides of the cylinder. (It is argued that such equilibria might
exist for vortex flows past a cylinder confined in a channel.) In addition, a
new family of opposite-signed equilibria on the normal line is reported. The
stability analysis for antisymmetric perturbations is also carried out and it
shows that all equilibria are unstable in this case.Comment: 27 pages, 13 figures, to be published in Physics of Fluid
Bernoulli type polynomials on Umbral Algebra
The aim of this paper is to investigate generating functions for modification
of the Milne-Thomson's polynomials, which are related to the Bernoulli
polynomials and the Hermite polynomials. By applying the Umbral algebra to
these generating functions, we provide to deriving identities for these
polynomials
Radio Emission from the Composite Supernova Remnant G326.3-1.8 (MSH15-56)
High resolution radio observations of the composite supernova remnant (SNR)
G326.3-1.8 or MSH 15-56 with the Australia Telescope Compact Array show details
of both the shell and the bright plerion which is offset about 1/3 of the
distance from the center of the SNR to the shell. The shell appears to be
composed of thin filaments, typical of older shell SNRs. The central part of
the elongated plerion is composed of a bundle of parallel ridges which bulge
out at the ends and form a distinct ring structure on the northwestern end. The
magnetic field with a strength of order 45 microGauss, is directed along the
axis of the ridges but circles around the northwestern ring. This plerion is
large and bright in the radio but is not detected in x-ray or optical
wavelengths. There is, however, a faint hard x-ray feature closer to the shell
outside the plerion. Perhaps if the supernova explosion left a rapidly moving
magnetar with large energy input but initially rapid decay of both relativistic
particles and magnetic field, the observed differences with wavelength could be
explained.Comment: 15 pages, 10 figures, accepted by Ap
Evidence of a Curved Synchrotron Spectrum in the Supernova Remnant SN 1006
A joint spectral analysis of some Chandra ACIS X-ray data and Molonglo
Observatory Synthesis Telescope radio data was performed for 13 small regions
along the bright northeastern rim of the supernova remnant SN 1006. These data
were fitted with a synchrotron radiation model. The nonthermal electron
spectrum used to compute the photon emission spectra is the traditional
exponentially cut off power law, with one notable difference: The power-law
index is not a constant. It is a linear function of the logarithm of the
momentum. This functional form enables us to show, for the first time, that the
synchrotron spectrum of SN 1006 seems to flatten with increasing energy. The
effective power-law index of the electron spectrum is 2.2 at 1 GeV (i.e., radio
synchrotron-emitting momenta) and 2.0 at about 10 TeV (i.e., X-ray
synchrotron-emitting momenta). This amount of change in the index is
qualitatively consistent with theoretical models of the amount of curvature in
the proton spectrum of the remnant. The evidence of spectral curvature implies
that cosmic rays are dynamically important instead of being "test" particles.
The spectral analysis also provides a means of determining the critical
frequency of the synchrotron spectrum associated with the highest-energy
electrons. The critical frequency seems to vary along the northeastern rim,
with a maximum value of 1.1e17 (0.6e17 - 2.1e17) Hz. This value implies that
the electron diffusion coefficient can be no larger than a factor of ~4.5-21
times the Bohm diffusion coefficient if the velocity of the forward shock is in
the range 2300-5000 km/s. Since the coefficient is close to the Bohm limit,
electrons are accelerated nearly as fast as possible in the regions where the
critical frequency is about 1.0e17 Hz.Comment: 41 pages, 8 figures, accepted by Ap
Radio Observations of the Supernova Remnant Candidate G312.5-3.0
The radio images from the Parkes-MIT-NRAO (PMN) Southern Sky Survey at 4850
MHz have revealed a number of previously unknown radio sources. One such
source, G312.5-3.0 (PMN J1421-6415), has been observed using the
multi-frequency capabilities of the Australia Telescope Compact Array (ATCA) at
frequencies of 1380 MHz and 2378 MHz. Further observations of the source were
made using the Molonglo Observatory Synthesis Telescope (MOST) at a frequency
of 843 MHz. The source has an angular size of 18 arcmin and has a distinct
shell structure. We present the reduced multi-frequency observations of this
source and provide a brief argument for its possible identification as a
supernova remnant.Comment: 5 pages, 5 figures, Accepted for publication in MNRA
A high-resolution radio survey of the Vela supernova remnant
This paper presents a high-resolution radio continuum (843 MHz) survey of the
Vela supernova remnant. The contrast between the structures in the central
pulsar-powered nebula of the remnant and the synchrotron radiation shell allows
the remnant to be identified morphologically as a member of the composite
class. The data are the first of a composite remnant at spatial scales
comparable with those available for the Cygnus Loop and the Crab Nebula, and
make possible a comparison of radio, optical and soft X-ray emission from the
resolved shell filaments. The survey, made with the Molonglo Observatory
Synthesis Telescope, covers an area of 50 square degrees at a resolution of
43'' x 60'', while imaging structures on scales up to 30'.Comment: 18 pages, 7 jpg figures (version with ps figures at
http://astro.berkeley.edu/~dbock/papers/); AJ, in pres
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