217 research outputs found
Regularization of fields for self-force problems in curved spacetime: foundations and a time-domain application
We propose an approach for the calculation of self-forces, energy fluxes and
waveforms arising from moving point charges in curved spacetimes. As opposed to
mode-sum schemes that regularize the self-force derived from the singular
retarded field, this approach regularizes the retarded field itself. The
singular part of the retarded field is first analytically identified and
removed, yielding a finite, differentiable remainder from which the self-force
is easily calculated. This regular remainder solves a wave equation which
enjoys the benefit of having a non-singular source. Solving this wave equation
for the remainder completely avoids the calculation of the singular retarded
field along with the attendant difficulties associated with numerically
modeling a delta function source. From this differentiable remainder one may
compute the self-force, the energy flux, and also a waveform which reflects the
effects of the self-force. As a test of principle, we implement this method
using a 4th-order (1+1) code, and calculate the self-force for the simple case
of a scalar charge moving in a circular orbit around a Schwarzschild black
hole. We achieve agreement with frequency-domain results to ~ 0.1% or better.Comment: 15 pages, 12 figures, 1 table. More figures, extended summar
Assessing the Effectiveness of Schools to Safeguard Their Pupil’s Use of Social Media Through an Analysis of School Inspection Reports
While social media remains a facet of life that many children and young people happily engage in, its use comes with some recognised risks and dangers. Parents can feel ill-equipped to support their children with this and there is consequently a reliance on schools to teach children how to use these platforms appropriately. To evaluate the effectiveness of secondary schools in teaching pupils about social media, this study makes use of evidence from Ofsted school inspection reports. Exploiting techniques developed in computer science, the Ofsted web portal was automatically scraped for reports and the content searched for reference to social media. This identified 317 reports which referred to the platforms. The report’s texts were coded through content analysis and subsequently revealed that over 90% of the references to social media contained in inspection reports were positive in reporting that pupils both understood the risks and knew how to describe how to manage their online activities. The results suggest that schools are effective in addressing these safeguarding issues although pupils are not always putting their knowledge into practise
Spectral Function for the S=1 Heisenberg Antiferromagetic Chain
We study the spectral function, for the spin-1, one dimensional
antiferromagnetic chain using a time-dependent density matrix renormalizaton
group (DMRG) numerical method. We develop methods for extrapolating the time
dependent correlation functions to larger times in order to enhance the
frequency resolution. The resulting spectral functions are impressively precise
and accurate. Our results confirm many qualitative expectations from non-linear
model methods and test them quantitatively. The critical wave-vector
at which the single particle excitation emerges from the 2-particle continuum
is estimated to be .Comment: 12 pages, 19 fig
Does sub-cluster merging accelerate mass segregation in local star formation?
The nearest site of massive star formation in Orion is dominated by the
Trapezium subsystem, with its four OB stars and numerous companions. The
question of how these stars came to be in such close proximity has implications
for our understanding of massive star formation and early cluster evolution. A
promising route toward rapid mass segregation was proposed by McMillan et al.
(2007), who showed that the merger product of faster-evolving sub clusters can
inherit their apparent dynamical age from their progenitors. In this paper we
briefly consider this process at a size and time scale more suited for local
and perhaps more typical star formation, with stellar numbers from the hundreds
to thousands. We find that for reasonable ages and cluster sizes, the merger of
sub-clusters can indeed lead to compact configurations of the most massive
stars, a signal seen both in Nature and in large-scale hydrodynamic simulations
of star formation from collapsing molecular clouds, and that sub-virial initial
conditions can make an un-merged cluster display a similar type of mass
segregation. Additionally, we discuss a variation of the minimum spanning tree
mass-segregation technique introduced by Allison et al. (2009).Comment: 9 pages, submitted to MNRA
SHELS: Testing Weak Lensing Maps with Redshift Surveys
Weak lensing surveys are emerging as an important tool for the construction
of "mass selected" clusters of galaxies. We evaluate both the efficiency and
completeness of a weak lensing selection by combining a dense, complete
redshift survey, the Smithsonian Hectospec Lensing Survey (SHELS), with a weak
lensing map from the Deep Lens Survey (DLS). SHELS includes 11,692 redshifts
for galaxies with R < 20.6 in the four square degree DLS field; the survey is a
solid basis for identifying massive clusters of galaxies with redshift z <
0.55. The range of sensitivity of the redshift survey is similar to the range
for the DLS convergence map. Only four the twelve convergence peaks with
signal-to-noise > 3.5 correspond to clusters of galaxies with M > 1.7 x 10^14
solar masses. Four of the eight massive clusters in SHELS are detected in the
weak lensing map yielding a completeness of roughly 50%. We examine the seven
known extended cluster x-ray sources in the DLS field: three can be detected in
the weak lensing map, three should not be detected without boosting from
superposed large-scale structure, and one is mysteriously undetected even
though its optical properties suggest that it should produce a detectable
lensing signal. Taken together, these results underscore the need for more
extensive comparisons among different methods of massive cluster
identification.Comment: 34 pages, 16 figures, ApJ accepte
Chebyshev matrix product state approach for spectral functions
We show that recursively generated Chebyshev expansions offer numerically
efficient representations for calculating zero-temperature spectral functions
of one-dimensional lattice models using matrix product state (MPS) methods. The
main features of this Chebychev matrix product state (CheMPS) approach are: (i)
it achieves uniform resolution over the spectral function's entire spectral
width; (ii) it can exploit the fact that the latter can be much smaller than
the model's many-body bandwidth; (iii) it offers a well-controlled broadening
scheme; (iv) it is based on a succession of Chebychev vectors |t_n>, (v) whose
entanglement entropies were found to remain bounded with increasing recursion
order n for all cases analyzed here; (vi) it distributes the total entanglement
entropy that accumulates with increasing n over the set of Chebyshev vectors
|t_n>. We present zero-temperature CheMPS results for the structure factor of
spin-1/2 antiferromagnetic Heisenberg chains and perform a detailed finite-size
analysis. Making comparisons to three benchmark methods, we find that CheMPS
(1) yields results comparable in quality to those of correction vector DMRG, at
dramatically reduced numerical cost; (2) agrees well with Bethe Ansatz results
for an infinite system, within the limitations expected for numerics on finite
systems; (3) can also be applied in the time domain, where it has potential to
serve as a viable alternative to time-dependent DMRG (in particular at finite
temperatures). Finally, we present a detailed error analysis of CheMPS for the
case of the noninteracting resonant level model.Comment: 22 pages, 13 figure
A New 24 micron Phase Curve for upsilon Andromedae b
We report the detection of 24 micron variations from the planet-hosting
upsilon Andromedae system consistent with the orbital periodicity of the
system's innermost planet, upsilon And b. We find a peak-to-valley phase curve
amplitude of 0.00130 times the mean system flux. Using a simple model with two
hemispheres of constant surface brightness and assuming a planetary radius of
1.3 Jupiter radii gives a planetary temperature contrast of >900 K and an
orbital inclination of >28 degrees. We further report the largest phase offset
yet observed for an extrasolar planet: the flux maximum occurs ~80 degrees
before phase 0.5. Such a large phase offset is difficult to reconcile with most
current atmospheric circulation models. We improve on earlier observations of
this system in several important ways: (1) observations of a flux calibrator
star demonstrate the MIPS detector is stable to 10^-4 on long timescales, (2)
we note that the background light varies systematically due to spacecraft
operations, precluding use of this background as a flux calibrator (stellar
flux measured above the background is not similarly affected), and (3) we
calibrate for flux variability correlated with motion of the star on the MIPS
detector. A reanalysis of our earlier observations of this system is consistent
with our new result.Comment: Submitted to ApJ. 15 pages, 6 figures, 4 table
X-ray observations of the edge-on star-forming galaxy NGC891 and its supernova SN1986J
We present XMM-Newton observations of NCG891, a nearby edge-on spiral galaxy.
We analyse the extent of the diffuse emission emitted from the disk of the
galaxy, and find that it has a single temperature profile with best fitting
temperature of 0.26 keV, though the fit of a dual-temperature plasma with
temperatures of 0.08 and 0.30 keV is also an acceptable fit. There is a
considerable amount of diffuse X-ray emission protruding from the disk in the
NW direction out to approximately 6 kpc. We analyse the point source population
using a Chandra observation, using a maximum likelihood method to find that the
slope of the cumulative luminosity function of point sources in the galaxy is
-0.77^{+0.13}_{-0.1}. Using a sample of other local galaxies, we compare the
X-ray and infrared properties of NGC891 with those of 'normal' and starburst
spiral galaxies, and conclude that NGC891 is most likely a starburst galaxy in
a quiescent state. We establish that the diffuse X-ray luminosity of spirals
scales with the far infra-red luminosity as L_X \propto L_FIR ^{0.87 +- 0.07},
except for extreme starbursts, and NGC891 does not fall in the latter category.
We study the supernova SN1986J in both XMM-Newton and Chandra observations, and
find that the X-ray luminosity has been declining with time more steeply than
expected (L_X \propto t^-3).Comment: 12 pages, 11 figures, accepted for publication in MNRA
1985: Abilene Christian College Bible Lectures - Full Text
WHAT THE CHURCH NEEDS TO HEAR
Being the Abilene Christian University Annual Bible Lectures 1985
Published by A-C-U PRESS
ACU Station Abilene, Texas 7969
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