47,801 research outputs found
Cities in fiction: Perambulations with John Berger
This paper explores selected novels by John Berger in which cities play a central role. These cities are places, partially real and partially imagined, where memory, hope, and despair intersect. My reading of the novels enables me to trace important themes in recent discourses on the nature of contemporary capitalism, including notions of resistance and universality. I also show how Berger?s work points to a writing that can break free from the curious capacity of capitalism to absorb and feed of its critique
Competing charge density waves and temperature-dependent nesting in 2H-TaSe2
Multiple charge density wave (CDW) phases in 2H-TaSe2 are investigated by
high-resolution synchrotron x-ray diffraction. In a narrow temperature range
immediately above the commensurate CDW transition, we observe a multi-q
superstructure with coexisting commensurate and incommensurate order
parameters, clearly distinct from the fully incommensurate state at higher
temperatures. This multi-q ordered phase, characterized by a temperature
hysteresis, is found both during warming and cooling, in contrast to previous
reports. In the normal state, the incommensurate superstructure reflection
gives way to a broad diffuse peak that persists nearly up to room temperature.
Its position provides a direct and accurate estimate of the Fermi surface
nesting vector, which evolves non-monotonically and approaches the commensurate
position as the temperature is increased. This behavior agrees with our recent
observations of the temperature-dependent Fermi surface in the same compound
[Phys. Rev. B 79, 125112 (2009)]
Broad-band Modeling of GRB Afterglows
Observations of GRB afterglows ranging from radio to X-ray frequencies
generate large data sets. Careful analysis of these broad-band data can give us
insight into the nature of the GRB progenitor population by yielding such
information like the total energy of the burst, the geometry of the fireball
and the type of environment into which the GRB explodes. We illustrate, by
example, how global, self-consistent fits are a robust approach for
characterizing the afterglow emission. This approach allows a relatively simple
comparison of different models and a way to determine the strengths and
weaknesses of these models, since all are treated self-consistently. Here we
quantify the main differences between the broad-band, self-consistent approach
and the traditional approach, using GRB000301C and GRB970508 as test cases.Comment: Appears in "Gamma-Ray Bursts in the Afterglow Era" proceedings of the
Roma 2000 GRB Workshop; 3 pages; 2 figure
On the Approximation Performance of Fictitious Play in Finite Games
We study the performance of Fictitious Play, when used as a heuristic for
finding an approximate Nash equilibrium of a 2-player game. We exhibit a class
of 2-player games having payoffs in the range [0,1] that show that Fictitious
Play fails to find a solution having an additive approximation guarantee
significantly better than 1/2. Our construction shows that for n times n games,
in the worst case both players may perpetually have mixed strategies whose
payoffs fall short of the best response by an additive quantity 1/2 -
O(1/n^(1-delta)) for arbitrarily small delta. We also show an essentially
matching upper bound of 1/2 - O(1/n)
A VALIDATION STUDY OF THE ROBERTSON-BERGER METER
An evaluation of the Robertson-Berger meter was done in order to address the question of whether instrumental and calibration factors can cause what might be interpreted as a change in the ground level solar ultraviolet-B (UV-B) flux. The evaluation consisted of reviewing information about the instrumentation and components in the published literature, a review of the records and procedures in both operations and calibrations, and examination of two instruments including temperature tests of them. It is shown that the instrument is basically stable and that the calibration procedures did not support data drift. There is a slight dependance of the two instruments upon temperature, 0.3°C/y and 0.6°C/y, which is not sufficient to lead to the reported UV-B trends of the order of 0.7 %/y. There is negligible temperature drift in the control electronics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73331/1/j.1751-1097.1992.tb09614.x.pd
The Angular Size and Proper Motion of the Afterglow of GRB 030329
The bright, nearby (z=0.1685) gamma-ray burst of 29 March 2003 has presented
us with the first opportunity to directly image the expansion of a GRB. This
burst reached flux density levels at centimeter wavelengths more than 50 times
brighter than any previously studied event. Here we present the results of a
VLBI campaign using the VLBA, VLA, Green Bank, Effelsberg, Arecibo, and
Westerbork telescopes that resolves the radio afterglow of GRB 030329 and
constrains its rate of expansion. The size of the afterglow is found to be
\~0.07 mas (0.2 pc) 25 days after the burst, and 0.17 mas (0.5 pc) 83 days
after the burst, indicating an average velocity of 3-5 c. This expansion is
consistent with expectations of the standard fireball model. We measure the
projected proper motion of GRB 030329 in the sky to <0.3 mas in the 80 days
following the burst. In observations taken 52 days after the burst we detect an
additional compact component at a distance from the main component of 0.28 +/-
0.05 mas (0.80 pc). The presence of this component is not expected from the
standard model.Comment: 12 pages including 2 figures, LaTeX. Accepted to ApJ Letters on May
14, 200
A Radio Flare from GRB 020405: Evidence for a Uniform Medium Around a Massive Stellar Progenitor
We present radio observations of GRB 020405 starting 1.2 days after the
burst, which reveal a rapidly-fading ``radio flare''. Based on its temporal and
spectral properties, we interpret the radio flare as emission from the reverse
shock. This scenario rules out a circumburst medium with a radial density
profile \rho ~ r^{-2} expected around a mass-losing massive star, since in that
case the reverse shock emission decays on the timescale of the burst duration
t~100 s. Using published optical and X-ray data, along with the radio data
presented here, we further show that a self-consistent model requires
collimated ejecta with an opening angle of 6 degrees (t_j~0.95 days). As a
consequence of the early jet break, the late-time (t>10 days) emission measured
with the Hubble Space Telescope significantly deviates from an extrapolation of
the early, ground-based data. This, along with an unusually red spectrum, F_\nu
\~ \nu^{-3.9}, strengthens the case for a supernova that exploded at about the
same time as GRB 020405, thus pointing to a massive stellar progenitor for this
burst. This is the first clear association of a massive progenitor with a
uniform medium, indicating that a \rho ~ r^{-2} profile is not a required
signature, and in fact may not be present on the lengthscales probed by the
afterglow in the majority of bursts.Comment: Submitted to ApJ; 14 pages, 2 tables, 3 figure
Superfield Realizations of Lorentz and CPT Violation
Superfield realizations of Lorentz-violating extensions of the Wess-Zumino
model are presented. These models retain supersymmetry but include terms that
explicitly break the Lorentz symmetry. The models can be understood as arising
from superspace transformations that are modifications of the familiar one in
the Lorentz-symmetric case.Comment: 10 page
Manufacture of Gowdy spacetimes with spikes
In numerical studies of Gowdy spacetimes evidence has been found for the
development of localized features (spikes) involving large gradients near the
singularity. The rigorous mathematical results available up to now did not
cover this kind of situation. In this work we show the existence of large
classes of Gowdy spacetimes exhibiting features of the kind discovered
numerically. These spacetimes are constructed by applying certain
transformations to previously known spacetimes without spikes. It is possible
to control the behaviour of the Kretschmann scalar near the singularity in
detail. This curvature invariant is found to blow up in a way which is
non-uniform near the spike in some cases. When this happens it demonstrates
that the spike is a geometrically invariant feature and not an artefact of the
choice of variables used to parametrize the metric. We also identify another
class of spikes which are artefacts. The spikes produced by our method are
compared with the results of numerical and heuristic analyses of the same
situation.Comment: 25 page
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