19,452 research outputs found
Engineering adiabaticity at an avoided crossing with optimal control
We investigate ways to optimize adiabaticity and diabaticity in the
Landau-Zener model with non-uniform sweeps. We show how diabaticity can be
engineered with a pulse consisting of a linear sweep augmented by an
oscillating term. We show that the oscillation leads to jumps in populations
whose value can be accurately modeled using a model of multiple,
photon-assisted Landau-Zener transitions, which generalizes work by Wubs et al.
[New J. Phys. 7, 218 (2005)]. We extend the study on diabaticity using methods
derived from optimal control. We also show how to preserve adiabaticity with
optimal pulses at limited time, finding a non-uniform quantum speed limit
Resolving the virial discrepancy in clusters of galaxies with modified Newtonian dynamics
A sample of 197 X-ray emitting clusters of galaxies is considered in the
context of Milgrom's modified Newtonian dynamics (MOND). It is shown that the
gas mass, extrapolated via an assumed model to a fixed radius of 3 Mpc,
is correlated with the gas temperature as predicted by MOND (). The observed temperatures are generally consistent with the inferred
mass of hot gas; no substantial quantity of additional unseen matter is
required in the context of MOND. However, modified dynamics cannot resolve the
strong lensing discrepancy in those clusters where this phenomenon occurs. The
prediction is that additional baryonic matter may be detected in the central
regions of rich clusters.Comment: Submitted to A&A, 4 pages, 3 figures, A&A macro
Implications of the first detection of coherent elastic neutrino-nucleus scattering (CEvNS) with Liquid Argon
The CENNS-10 experiment of the COHERENT collaboration has recently reported
the first detection of coherent-elastic neutrino-nucleus scattering (CEvNS) in
liquid Argon with more than significance. In this work, we exploit
the new data in order to probe various interesting parameters which are of key
importance to CEvNS within and beyond the Standard Model. A dedicated
statistical analysis of these data shows that the current constraints are
significantly improved in most cases. We derive a first measurement of the
neutron rms charge radius of Argon, and also an improved determination of the
weak mixing angle in the low energy regime. We also update the constraints on
neutrino non-standard interactions, electromagnetic properties and light
mediators with respect to those derived from the first COHERENT-CsI data.Comment: discussion expanded including light mediators and nuclear
uncertainties, figures added, references added. V3: Fig. 7 corrected,
conclusions unchange
Bayes-X: a Bayesian inference tool for the analysis of X-ray observations of galaxy clusters
We present the first public release of our Bayesian inference tool, Bayes-X,
for the analysis of X-ray observations of galaxy clusters. We illustrate the
use of Bayes-X by analysing a set of four simulated clusters at z=0.2-0.9 as
they would be observed by a Chandra-like X-ray observatory. In both the
simulations and the analysis pipeline we assume that the dark matter density
follows a spherically-symmetric Navarro, Frenk and White (NFW) profile and that
the gas pressure is described by a generalised NFW (GNFW) profile. We then
perform four sets of analyses. By numerically exploring the joint probability
distribution of the cluster parameters given simulated Chandra-like data, we
show that the model and analysis technique can robustly return the simulated
cluster input quantities, constrain the cluster physical parameters and reveal
the degeneracies among the model parameters and cluster physical parameters. We
then analyse Chandra data on the nearby cluster, A262, and derive the cluster
physical profiles. To illustrate the performance of the Bayesian model
selection, we also carried out analyses assuming an Einasto profile for the
matter density and calculated the Bayes factor. The results of the model
selection analyses for the simulated data favour the NFW model as expected.
However, we find that the Einasto profile is preferred in the analysis of A262.
The Bayes-X software, which is implemented in Fortran 90, is available at
http://www.mrao.cam.ac.uk/facilities/software/bayesx/.Comment: 22 pages, 11 figure
Properties of galaxy dark matter halos from weak lensing
We present the results of a study of weak lensing by galaxies based on 45.5
deg of band imaging data from the Red-Sequence Cluster Survey (RCS).
We present the first weak lensing detection of the flattening of galaxy dark
matter halos. We use a simple model in which the ellipticity of the halo is
times the observed ellipticity of the lens. We find a best fit value of
, suggesting that the dark matter halos are somewhat
rounder than the light distribution. The fact that we detect a significant
flattening implies that the halos are well aligned with the light distribution.
Given the average ellipticity of the lenses, this implies a halo ellipticity of
, in fair agreement with results from
numerical simulations of CDM. This result provides strong support for the
existence of dark matter, as an isotropic lensing signal is excluded with 99.5%
confidence. We also study the average mass profile around the lenses, using a
maximum likelihood analysis. We consider two models for the halo mass profile:
a truncated isothermal sphere (TIS) and an NFW profile. We adopt
observationally motivated scaling relations between the lens luminosity and the
velocity dispersion and the extent of the halo. The best fit NFW model yields a
mass and a scale
radius kpc. This value for the scale radius is
in excellent agreement with predictions from numerical simulations for a halo
of this mass.Comment: Significantly revised version, accepted for publication in ApJ 11
pages, 6 figure
Growing supermassive black holes in the late stages of galaxy mergers are heavily obscured
Mergers of galaxies are thought to cause significant gas inflows to the inner
parsecs, which can activate rapid accretion onto supermassive black holes
(SMBHs), giving rise to Active Galactic Nuclei (AGN). During a significant
fraction of this process, SMBHs are predicted to be enshrouded by gas and dust.
Studying 52 galactic nuclei in infrared-selected local Luminous and
Ultra-luminous infrared galaxies in different merger stages in the hard X-ray
band, where radiation is less affected by absorption, we find that the amount
of material around SMBHs increases during the last phases of the merger. We
find that the fraction of Compton-thick (CT, ) AGN in late merger galaxies is higher
() than in local hard X-ray selected AGN
(), and that obscuration reaches its maximum when the
nuclei of the two merging galaxies are at a projected distance of
kiloparsecs (). We also
find that all AGN of our sample in late merger galaxies have , which implies that the obscuring material covers
of the X-ray source. These observations show that the material
is most effectively funnelled from the galactic scale to the inner tens of
parsecs during the late stages of galaxy mergers, and that the close
environment of SMBHs in advanced mergers is richer in gas and dust with respect
to that of SMBHs in isolated galaxies, and cannot be explained by the classical
AGN unification model in which the torus is responsible for the obscuration.Comment: Final version matching the article published in MNRAS - 30 pages, 16
figure
Calcification in a marginal sea - influence of seawater [Ca2+] and carbonate chemistry on bivalve shell formation
In estuarine coastal systems such as the Baltic Sea, mussels suffer from low salinity which limits their distribution. Anthropogenic climate change is expected to cause further desalination which will lead to local extinctions of mussels in the low saline areas. It is commonly accepted that mussel distribution is limited by osmotic stress. However, along the salinity gradient environmental conditions for biomineralization are successively becoming more adverse as a result of reduced [Ca2+] and dissolved inorganic carbon (CT) availability. In larvae, calcification is an essential process starting during early development with formation of the prodissoconch I (PD I) shell which is completed under optimal conditions within 2 days.
Experimental manipulations of seawater [Ca2+] start to impair PD I formation in Mytilus larvae at concentrations below 3 mM, which corresponds to conditions present in the Baltic at salinities below 8 g kg-1. In addition, lowering dissolved inorganic carbon to critical concentrations (< 1 mM) similarly affected PD I size which was well correlated with calculated ΩAragonite and [Ca2+][HCO3-]/[H+] in all treatments. Comparing results for larvae from the western Baltic with a population from the central Baltic revealed significantly higher tolerance of PD I formation to lowered [Ca2+] and [Ca2+][HCO3-]/[H+] in the low saline adapted population. This may result from genetic adaptation to the more adverse environmental conditions prevailing in the low saline areas of the Baltic.
The combined effects of lowered [Ca2+] and adverse carbonate chemistry represent major limiting factors for bivalve calcification and can thereby contribute to distribution limits of mussels in the Baltic Sea
Geometric phase distributions for open quantum systems
In an open system, the geometric phase should be described by a distribution.
We show that a geometric phase distribution for open system dynamics is in
general ambiguous, but the imposition of reasonable physical constraints on the
environment and its coupling with the system yields a unique geometric phase
distribution that applies even for mixed states, non-unitary dynamics, and
non-cyclic evolutions.Comment: Some minor revisions, references update
Index selection of beef cattle for growth and milk production using computer simulation modelling
The Texas A&M University (TAMU) Beef Cattle Production model was expanded to include basic concepts of quantitative genetics. The traits simulated were birth weight, yearling weight, mature weight and milk production. The progeny inherited attributes from both the sire and the dam. The incorporation of genetic concepts into the model allowed for the introduction of variation between individuals and generations. This was achieved by interfacing the original model with stochastic genetic subroutines including a restricted selection index for desired genetic change. The index included birth weight and yearling weight. In addition, a function for estimating dystocia was also added. The model was used to simulate selection strategies for a small-to-moderate size breed of cattle and a large-size breed using a restricted selection index.
There was an increase in both birth and yearling weight after 20 years of selection in the small-to-moderate size breed, and there was also an increase in dystocia. Antagonistic selection to decrease birth weight and increase yearling weight was simulated for the large breed. Birth weight and dystocia problems declined while yearling weight increased for all classes of animals. In both experiments correlated responses were recorded for mature size and milk production. It was concluded that the modified TAMU Beef Cattle Production model offers breeders an opportunity to compare different selection strategies and evaluate different breeding plans.
(South African Journal of Animal Science, 2001, 31(2): 65-76
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