33,989 research outputs found
Closed-form expressions for correlated density matrices: application to dispersive interactions and example of (He)2
Empirically correlated density matrices of N-electron systems are
investigated. Exact closed-form expressions are derived for the one- and
two-electron reduced density matrices from a general pairwise correlated wave
function. Approximate expressions are proposed which reflect dispersive
interactions between closed-shell centro-symmetric subsystems. Said expressions
clearly illustrate the consequences of second-order correlation effects on the
reduced density matrices. Application is made to a simple example: the (He)2
system. Reduced density matrices are explicitly calculated, correct to second
order in correlation, and compared with approximations of independent electrons
and independent electron pairs. The models proposed allow for variational
calculations of interaction energies and equilibrium distance as well as a
clear interpretation of dispersive effects on electron distributions. Both
exchange and second order correlation effects are shown to play a critical role
on the quality of the results.Comment: 22 page
Pathways to double ionization of atoms in strong fields
We discuss the final stages of double ionization of atoms in a strong
linearly polarized laser field within a classical model. We propose that all
trajectories leading to non-sequential double ionization pass close to a saddle
in phase space which we identify and characterize. The saddle lies in a two
degree of freedom subspace of symmetrically escaping electrons. The
distribution of longitudinal momenta of ions as calculated within the subspace
shows the double hump structure observed in experiments. Including a symmetric
bending mode of the electrons allows us to reproduce the transverse ion
momenta. We discuss also a path to sequential ionization and show that it does
not lead to the observed momentum distributions.Comment: 10 pages, 10 figures; fig.6 and 7 exchanged in the final version
accepted for publication in Phys. Rev.
The L_X--M relation of Clusters of Galaxies
We present a new measurement of the scaling relation between X-ray luminosity
and total mass for 17,000 galaxy clusters in the maxBCG cluster sample.
Stacking sub-samples within fixed ranges of optical richness, N_200, we measure
the mean 0.1-2.4 keV X-ray luminosity, , from the ROSAT All-Sky Survey.
The mean mass, , is measured from weak gravitational lensing of SDSS
background galaxies (Johnston et al. 2007). For 9 <= N_200 < 200, the data are
well fit by a power-law, /10^42 h^-2 erg/s = (12.6+1.4-1.3 (stat) +/- 1.6
(sys)) (/10^14 h^-1 M_sun)^1.65+/-0.13. The slope agrees to within 10%
with previous estimates based on X-ray selected catalogs, implying that the
covariance in L_X and N_200 at fixed halo mass is not large. The luminosity
intercent is 30%, or 2\sigma, lower than determined from the X-ray flux-limited
sample of Reiprich & Bohringer (2002), assuming hydrostatic equilibrium. This
difference could arise from a combination of Malmquist bias and/or systematic
error in hydrostatic mass estimates, both of which are expected. The intercept
agrees with that derived by Stanek et al. (2006) using a model for the
statistical correspondence between clusters and halos in a WMAP3 cosmology with
power spectrum normalization sigma_8 = 0.85. Similar exercises applied to
future data sets will allow constraints on the covariance among optical and hot
gas properties of clusters at fixed mass.Comment: 5 pages, 1 figure, MNRAS accepte
Non-sequential triple ionization in strong fields
We consider the final stage of triple ionization of atoms in a strong
linearly polarized laser field. We propose that for intensities below the
saturation value for triple ionization the process is dominated by the
simultaneous escape of three electrons from a highly excited intermediate
complex. We identify within a classical model two pathways to triple
ionization, one with a triangular configuration of electrons and one with a
more linear one. Both are saddles in phase space. A stability analysis
indicates that the triangular configuration has the larger cross sections and
should be the dominant one. Trajectory simulations within the dominant symmetry
subspace reproduce the experimentally observed distribution of ion momenta
parallel to the polarization axis.Comment: 9 pages, 8 figures, accepted for publication in Phys. Rev.
Thermal X-rays from Millisecond Pulsars: Constraining the Fundamental Properties of Neutron Stars
Abridged) We model the X-ray properties of millisecond pulsars (MSPs) by
considering hot spot emission from a weakly magnetized rotating neutron star
(NS) covered by an optically-thick hydrogen atmosphere. We investigate the
limitations of using the thermal X-ray pulse profiles of MSPs to constrain the
mass-to-radius () ratio of the underlying NS. The accuracy is strongly
dependent on the viewing angle and magnetic inclination. For certain systems,
the accuracy is ultimately limited only by photon statistics implying that
future X-ray observatories could, in principle, achieve constraints on
and hence the NS equation of state to better than 5%. We demonstrate that
valuable information regarding the basic properties of the NS can be extracted
even from X-ray data of fairly limited photon statistics through modeling of
archival spectroscopic and timing observations of the nearby isolated PSRs
J0030+0451 and J2124--3358. The X-ray emission from these pulsars is consistent
with the presence of a hydrogen atmosphere and a dipolar magnetic field
configuration, in agreement with previous findings for PSR J0437--4715. For
both MSPs, the favorable geometry allows us to place interesting limits on the
allowed of NSs. Assuming 1.4 M, the stellar radius is
constrained to be km and km (68% confidence) for PSRs
J0030+0451 and J2124--3358, respectively. We explore the prospects of using
future observatories such as \textit{Constellation-X} and \textit{XEUS} to
conduct blind X-ray timing searches for MSPs not detectable at radio
wavelengths due to unfavorable viewing geometry. Using the observational
constraints on the pulsar obliquities we are also able to place strong
constraints on the magnetic field evolution model proposed by Ruderman.Comment: 9 pages, 7 figures, published in the Astrophysical Journal (Volume
689, Issue 1, pp. 407-415
The Mean and Scatter of the Velocity Dispersion-Optical Richness Relation for maxBCG Galaxy Clusters
The distribution of galaxies in position and velocity around the centers of
galaxy clusters encodes important information about cluster mass and structure.
Using the maxBCG galaxy cluster catalog identified from imaging data obtained
in the Sloan Digital Sky Survey, we study the BCG-galaxy velocity correlation
function. By modeling its non-Gaussianity, we measure the mean and scatter in
velocity dispersion at fixed richness. The mean velocity dispersion increases
from 202+/-10 km/s for small groups to more than 854+/-102 km/s for large
clusters. We show the scatter to be at most 40.5+/-3.5%, declining to
14.9+/-9.4% in the richest bins. We test our methods in the C4 cluster catalog,
a spectroscopic cluster catalog produced from the Sloan Digital Sky Survey DR2
spectroscopic sample, and in mock galaxy catalogs constructed from N-body
simulations. Our methods are robust, measuring the scatter to well within
one-sigma of the true value, and the mean to within 10%, in the mock catalogs.
By convolving the scatter in velocity dispersion at fixed richness with the
observed richness space density function, we measure the velocity dispersion
function of the maxBCG galaxy clusters. Although velocity dispersion and
richness do not form a true mass-observable relation, the relationship between
velocity dispersion and mass is theoretically well characterized and has low
scatter. Thus our results provide a key link between theory and observations up
to the velocity bias between dark matter and galaxies.Comment: 25 pages, 15 figures, 2 tables, published in Ap
Is pulsar B0656+14 a very nearby RRAT source?
The recently discovered RRAT sources are characterized by very bright radio
bursts which, while being periodically related, occur infrequently. We find
bursts with the same characteristics for the known pulsar B0656+14. These
bursts represent pulses from the bright end of an extended smooth pulse-energy
distribution and are shown to be unlike giant pulses, giant micropulses or the
pulses of normal pulsars. The extreme peak-fluxes of the brightest of these
pulses indicates that PSR B0656+14, were it not so near, could only have been
discovered as an RRAT source. Longer observations of the RRATs may reveal that
they, like PSR B0656+14, emit weaker emission in addition to the bursts.Comment: 4 pages, 4 figures, accepted by ApJ
Low-temperature ordered phases of the spin- XXZ chain system CsCoCl
In this study the magnetic order of the spin-1/2 XXZ chain system
CsCoCl in a temperature range from 50 mK to 0.5 K and in applied
magnetic fields up to 3.5 T is investigated by high-resolution measurements of
the thermal expansion and the specific heat. Applying magnetic fields along a
or c suppresses completely at about 2.1 T. In addition, we find
an adjacent intermediate phase before the magnetization saturates close to 2.5
T. For magnetic fields applied along b, a surprisingly rich phase diagram
arises. Two additional transitions are observed at critical fields T and T, which we propose to
arise from a two-stage spin-flop transition.Comment: 10 pages, 10 figure
Direct measurement of the 14N(p,g)15O S-factor
We have measured the 14N(p,g)15O excitation function for energies in the
range E_p = 155--524 keV. Fits of these data using R-matrix theory yield a
value for the S-factor at zero energy of 1.64(17) keV b, which is significantly
smaller than the result of a previous direct measurement. The corresponding
reduction in the stellar reaction rate for 14N(p,g)15O has a number of
interesting consequences, including an impact on estimates for the age of the
Galaxy derived from globular clusters.Comment: 5 pages, 3 figures, submitted to Phys. Rev. Let
Three-Dimensional Supergravity and the Cosmological Constant
Witten has argued that in dimensions local supersymmetry can ensure the
vanishing of the cosmological constant without requiring the equality of bose
and fermi masses. We find that this mechanism is implemented in a novel fashion
in the (2+1)-dimensional supersymmetric abelian Higgs model coupled to
supergravity. The vortex solitons are annihilated by half of the supersymmetry
transformations. The covariantly constant spinors required to define these
supersymmetries exist by virtue of a surprising cancellation between the
Aharonov-Bohm phase and the phase associated with the holonomy of the spin
connection. However the other half of the supersymmetry transformations, whose
actions ordinarily generate the soliton supermultiplet, are not well-defined
and bose-fermi degeneracy is consequently absent in the soliton spectrum.Comment: 12 pages, phyzz
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