2,511 research outputs found
Delensing Gravitational Wave Standard Sirens with Shear and Flexion Maps
Supermassive black hole binary systems (SMBHB) are standard sirens -- the
gravitational wave analogue of standard candles -- and if discovered by
gravitational wave detectors, they could be used as precise distance
indicators. Unfortunately, gravitational lensing will randomly magnify SMBHB
signals, seriously degrading any distance measurements. Using a weak lensing
map of the SMBHB line of sight, we can estimate its magnification and thereby
remove some uncertainty in its distance, a procedure we call "delensing." We
find that delensing is significantly improved when galaxy shears are combined
with flexion measurements, which reduce small-scale noise in reconstructed
magnification maps. Under a Gaussian approximation, we estimate that delensing
with a 2D mosaic image from an Extremely Large Telescope (ELT) could reduce
distance errors by about 30-40% for a SMBHB at z=2. Including an additional
wide shear map from a space survey telescope could reduce distance errors by
50%. Such improvement would make SMBHBs considerably more valuable as
cosmological distance probes or as a fully independent check on existing
probes.Comment: 9 pages, 4 figures, submitted to MNRA
The Electrostatic Persistence Length Calculated from Monte Carlo, Variational and Perturbation Methods
Monte Carlo simulations and variational calculations using a Gaussian ansatz
are applied to a model consisting of a flexible linear polyelectrolyte chain as
well as to an intrinsically stiff chain with up to 1000 charged monomers.
Addition of salt is treated implicitly through a screened Coulomb potential for
the electrostatic interactions.
For the flexible model the electrostatic persistence length shows roughly
three regimes in its dependence on the Debye-H\"{u}ckel screening length,
.As long as the salt content is low and is longer
than the end-to-end distance, the electrostatic persistence length varies only
slowly with . Decreasing the screening length, a controversial
region is entered. We find that the electrostatic persistence length scales as
, in agreement with experiment on flexible
polyelectrolytes, where is a strength parameter measuring the
electrostatic interactions within the polyelectrolyte. For screening lengths
much shorter than the bond length, the dependence becomes
quadratic in the variational calculation. The simulations suffer from numerical
problems in this regime, but seem to give a relationship half-way between
linear and quadratic.
A low temperature expansion only reproduces the first regime and a high
temperature expansion, which treats the electrostatic interactions as a
perturbation to a Gaussian chain, gives a quadratic dependence on the Debye
length.
For a sufficiently stiff chain, the persistence length varies quadratically
with in agreement with earlier theories.Comment: 20 pages LaTeX, 9 postscript figure
Isotope Shifts in Beryllium-, Boron-, Carbon-, and Nitrogen-like Ions from Relativistic Configuration Interaction Calculations
Energy levels, normal and specific mass shift parameters as well as
electronic densities at the nucleus are reported for numerous states along the
beryllium, boron, carbon, and nitrogen isoelectronic sequences. Combined with
nuclear data, these electronic parameters can be used to determine values of
level and transition isotope shifts. The calculation of the electronic
parameters is done using first-order perturbation theory with relativistic
configuration interaction wave functions that account for valence, core-valence
and core-core correlation effects as zero-order functions. Results are compared
with experimental and other theoretical values, when available.Comment: 56 pages, 1 figure, Atomic Data and Nuclear Data Tables (2014
Large-scale Multiconfiguration Hartree-fock Calculations of Hyperfine-interaction Constants For Low-lying States In Beryllium, Boron, and Carbon
Multiconfiguration Hartree-Fock (MCHF) calculations of hyperfine constants for the 2s2p 3P states of beryllium and the ground states of boron and carbon are reported. The capacity of a recently developed configuration-interaction program [Froese Fischer and Tong (unpublished); Stathopoulos and Froese Fischer (unpublished)], allowing for large configuration expansions, is explored. Using a systematic active-space MCHF approach, combined with large multireference configuration-interaction calculations, it is shown that hyperfine constants can be calculated very accurately. To reliably account for spin-polarization of the Is and 2s shells in boron and carbon, three-particle effects had to be included in a systematic way. The relativistic, finite-nuclear-size- and finite-nuclear-mass-corrected values of the hyperfine constants are compared with experimental values and with the most accurate theoretical values obtained with other methods
A Variational Approach for Minimizing Lennard-Jones Energies
A variational method for computing conformational properties of molecules
with Lennard-Jones potentials for the monomer-monomer interactions is
presented. The approach is tailored to deal with angular degrees of freedom,
{\it rotors}, and consists in the iterative solution of a set of deterministic
equations with annealing in temperature. The singular short-distance behaviour
of the Lennard-Jones potential is adiabatically switched on in order to obtain
stable convergence. As testbeds for the approach two distinct ensembles of
molecules are used, characterized by a roughly dense-packed ore a more
elongated ground state. For the latter, problems are generated from natural
frequencies of occurrence of amino acids and phenomenologically determined
potential parameters; they seem to represent less disorder than was previously
assumed in synthetic protein studies. For the dense-packed problems in
particular, the variational algorithm clearly outperforms a gradient descent
method in terms of minimal energies. Although it cannot compete with a careful
simulating annealing algorithm, the variational approach requires only a tiny
fraction of the computer time. Issues and results when applying the method to
polyelectrolytes at a finite temperature are also briefly discussed.Comment: 14 pages, uuencoded compressed postscript fil
Constraining dark matter halo properties using lensed SNLS supernovae
This paper exploits the gravitational magnification of SNe Ia to measure
properties of dark matter haloes. The magnification of individual SNe Ia can be
computed using observed properties of foreground galaxies and dark matter halo
models. We model the dark matter haloes of the galaxies as truncated singular
isothermal spheres with velocity dispersion and truncation radius obeying
luminosity dependent scaling laws. A homogeneously selected sample of 175 SNe
Ia from the first 3-years of the Supernova Legacy Survey (SNLS) in the redshift
range 0.2 < z < 1 is used to constrain models of the dark matter haloes
associated with foreground galaxies. The best-fitting velocity dispersion
scaling law agrees well with galaxy-galaxy lensing measurements. We further
find that the normalisation of the velocity dispersion of passive and star
forming galaxies are consistent with empirical Faber-Jackson and Tully-Fisher
relations, respectively. If we make no assumption on the normalisation of these
relations, we find that the data prefer gravitational lensing at the 92 per
cent confidence level. Using recent models of dust extinction we deduce that
the impact of this effect on our results is very small. We also investigate the
brightness scatter of SNe Ia due to gravitational lensing. The gravitational
lensing scatter is approximately proportional to the SN Ia redshift. We find
the constant of proportionality to be B = 0.055 +0.039 -0.041 mag (B < 0.12 mag
at the 95 per cent confidence level). If this model is correct, the
contribution from lensing to the intrinsic brightness scatter of SNe Ia is
small for the SNLS sample.Comment: 11 pages, 7 figures, accepted for publication in MNRA
Extended calculations of energy levels, radiative properties, , hyperfine interaction constants, and Land\'e -factors for nitrogen-like \mbox{Ge XXVI}
Employing two state-of-the-art methods, multiconfiguration
Dirac--Hartree--Fock and second-order many-body perturbation theory, highly
accurate calculations are performed for the lowest 272 fine-structure levels
arising from the , , , ~(), (), and ()
configurations in nitrogen-like Ge XXVI. Complete and consistent atomic data,
including excitation energies, lifetimes, wavelengths, hyperfine structures,
Land\'e -factors, and E1, E2, M1, M2 line strengths, oscillator
strengths, and transition rates among these 272 levels are provided.
Comparisons are made between the present two data sets, as well as with other
available experimental and theoretical values. The present data are accurate
enough for identification and deblending of emission lines involving the
levels, and are also useful for modeling and diagnosing fusion plasmas
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