3,041 research outputs found
A New Estimate of the Hubble Time with Improved Modeling of Gravitational Lenses
This paper examines free-form modeling of gravitational lenses using Bayesian
ensembles of pixelated mass maps. The priors and algorithms from previous work
are clarified and significant technical improvements are made. Lens
reconstruction and Hubble Time recovery are tested using mock data from simple
analytic models and recent galaxy-formation simulations. Finally, using
published data, the Hubble Time is inferred through the simultaneous
reconstruction of eleven time-delay lenses. The result is
H_0^{-1}=13.7^{+1.8}_{-1.0} Gyr.Comment: 24 pages, 9 figures. Accepted to Ap
Method of fabricating a twisted composite superconductor
A method of producing a twisted, stabilized wire or tube superconductor which can be used to wind electromagnets, armatures, rotors, field windings for motors and generators, and other magnetic devices which use a solenoid, toroidal, or other type winding is reported. At least one groove is formed along the length of a wire substrate which is then twisted into a helix and a layer of intermetallic superconducting material is formed in the groove. This layer can be formed by depositing the desired intermetallic compound into the groove or by diffusing one component of the superconductor into the groove formed in a substrate composed of the other component. The superconductor prepared by this method comprises a non-superconductor wire twisted into the shape of a helix, having at least one groove containing a layer of superconductor material along the length of the wire
New twisted intermetallic compound superconductor: A concept
Method for processing Nb3Sn and other intermetallic compound superconductors produces a twisted, stabilized wire or tube which can be used to wind electromagnetics, armatures, rotors, and field windings for motors and generators as well as other magnetic devices
Modeling the impact of iron and phosphorus limitations on nitrogen fixation in the Atlantic Ocean
International audienceThe overarching goal of this study is to simulate subsurface N* (sensu, Gruber and Sarmiento, 1997) anomaly patterns in the North Atlantic Ocean and determine the basin wide rates of N2 fixation that are required to do so. We present results from an Atlantic implementation of a coupled physical-biogeochemical model that includes an explicit, dynamic representation of N2 fixation with light, nitrogen, phosphorus and iron limitations, and variable stoichiometric ratios. The model is able to reproduce nitrogen, phosphorus and iron concentration variability to first order. The latter is achieved by incorporating iron deposition directly into the model's detritus compartment which allows the model to reproduce sharp near surface gradients in dissolved iron concentration off the west coast of Africa and deep dissolved iron concentrations that have been observed in recent observational studies. The model can reproduce the large scale N* anomaly patterns but requires relatively high rates of surface nitrogen fixation to do so (1.8×1012 moles N yr?1 from 10° N?30° N, 3.4×1012 moles N y
A Counts-in-Cells Analysis of Lyman-break Galaxies at z~3
We have measured the counts-in-cells fluctuations of 268 Lyman-break galaxies
with spectroscopic redshifts in six 9 arcmin by 9 arcmin fields at z~3. The
variance of galaxy counts in cubes of comoving side length 7.7, 11.9, 11.4
h^{-1} Mpc is \sigma_{gal}^2 ~ 1.3\pm0.4 for \Omega_M=1, 0.2 open, 0.3 flat,
implying a bias on these scales of \sigma_{gal} / \sigma_{mass} = 6.0\pm1.1,
1.9\pm0.4, 4.0\pm0.7. The bias and abundance of Lyman-break galaxies are
surprisingly consistent with a simple model of structure formation which
assumes only that galaxies form within dark matter halos, that Lyman-break
galaxies' rest-UV luminosities are tightly correlated with their dark masses,
and that matter fluctuations are Gaussian and have a linear power-spectrum
shape at z~3 similar to that determined locally (\Gamma~0.2). This conclusion
is largely independent of cosmology or spectral normalization \sigma_8. A
measurement of the masses of Lyman-break galaxies would in principle
distinguish between different cosmological scenarios.Comment: Accepted for publication in ApJ, 16 pages including 4 figure
Theory of Parabolic Arcs in Interstellar Scintillation Spectra
Our theory relates the secondary spectrum, the 2D power spectrum of the radio
dynamic spectrum, to the scattered pulsar image in a thin scattering screen
geometry. Recently discovered parabolic arcs in secondary spectra are generic
features for media that scatter radiation at angles much larger than the rms
scattering angle. Each point in the secondary spectrum maps particular values
of differential arrival-time delay and fringe rate (or differential Doppler
frequency) between pairs of components in the scattered image. Arcs correspond
to a parabolic relation between these quantities through their common
dependence on the angle of arrival of scattered components. Arcs appear even
without consideration of the dispersive nature of the plasma. Arcs are more
prominent in media with negligible inner scale and with shallow wavenumber
spectra, such as the Kolmogorov spectrum, and when the scattered image is
elongated along the velocity direction. The arc phenomenon can be used,
therefore, to constrain the inner scale and the anisotropy of scattering
irregularities for directions to nearby pulsars. Arcs are truncated by finite
source size and thus provide sub micro arc sec resolution for probing emission
regions in pulsars and compact active galactic nuclei. Multiple arcs sometimes
seen signify two or more discrete scattering screens along the propagation
path, and small arclets oriented oppositely to the main arc persisting for long
durations indicate the occurrence of long-term multiple images from the
scattering screen.Comment: 22 pages, 11 figures, submitted to the Astrophysical Journa
Analyses of shocked quartz at the global K-P boundary indicate an origin from a single, high-angle, oblique impact at Chicxulub
Accepted versio
Modeling temporal fluctuations in avalanching systems
We demonstrate how to model the toppling activity in avalanching systems by
stochastic differential equations (SDEs). The theory is developed as a
generalization of the classical mean field approach to sandpile dynamics by
formulating it as a generalization of Itoh's SDE. This equation contains a
fractional Gaussian noise term representing the branching of an avalanche into
small active clusters, and a drift term reflecting the tendency for small
avalanches to grow and large avalanches to be constricted by the finite system
size. If one defines avalanching to take place when the toppling activity
exceeds a certain threshold the stochastic model allows us to compute the
avalanche exponents in the continum limit as functions of the Hurst exponent of
the noise. The results are found to agree well with numerical simulations in
the Bak-Tang-Wiesenfeld and Zhang sandpile models. The stochastic model also
provides a method for computing the probability density functions of the
fluctuations in the toppling activity itself. We show that the sandpiles do not
belong to the class of phenomena giving rise to universal non-Gaussian
probability density functions for the global activity. Moreover, we demonstrate
essential differences between the fluctuations of total kinetic energy in a
two-dimensional turbulence simulation and the toppling activity in sandpiles.Comment: 14 pages, 11 figure
Halo Clustering with Non-Local Non-Gaussianity
We show how the peak-background split can be generalized to predict the
effect of non-local primordial non-Gaussianity on the clustering of halos. Our
approach is applicable to arbitrary primordial bispectra. We show that the
scale-dependence of halo clustering predicted in the peak-background split
(PBS) agrees with that of the local-biasing model on large scales. On smaller
scales, k >~ 0.01 h/Mpc, the predictions diverge, a consequence of the
assumption of separation of scales in the peak-background split. Even on large
scales, PBS and local biasing do not generally agree on the amplitude of the
effect outside of the high-peak limit. The scale dependence of the biasing -
the effect that provides strong constraints to the local-model bispectrum - is
far weaker for the equilateral and self-ordering-scalar-field models of
non-Gaussianity. The bias scale dependence for the orthogonal and folded models
is weaker than in the local model (~ 1/k), but likely still strong enough to be
constraining. We show that departures from scale-invariance of the primordial
power spectrum may lead to order-unity corrections, relative to predictions
made assuming scale-invariance - to the non-Gaussian bias in some of these
non-local models for non-Gaussianity. An Appendix shows that a non-local model
can produce the local-model bispectrum, a mathematical curiosity we uncovered
in the course of this investigation.Comment: 12 pages, 4 figures; submitted to Phys. Rev. D; v2: references added;
v3: some more comments on kernel-bispectrum relation in appendi
Identifying short motifs by means of extreme value analysis
The problem of detecting a binding site -- a substring of DNA where
transcription factors attach -- on a long DNA sequence requires the recognition
of a small pattern in a large background. For short binding sites, the matching
probability can display large fluctuations from one putative binding site to
another. Here we use a self-consistent statistical procedure that accounts
correctly for the large deviations of the matching probability to predict the
location of short binding sites. We apply it in two distinct situations: (a)
the detection of the binding sites for three specific transcription factors on
a set of 134 estrogen-regulated genes; (b) the identification, in a set of 138
possible transcription factors, of the ones binding a specific set of nine
genes. In both instances, experimental findings are reproduced (when available)
and the number of false positives is significantly reduced with respect to the
other methods commonly employed.Comment: 6 pages, 5 figure
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