1,087 research outputs found
VLBA measurement of the transverse velocity of the magnetar XTE J1810-197
We have obtained observations of the magnetar XTE J1810-197 with the Very
Long Baseline Array at two epochs separated by 106 days, at wavelengths of 6 cm
and 3.6 cm. Comparison of the positions yields a proper motion value of
13.5+-1.0 mas/yr at an equatorial position angle of 209.4+-2.4 deg (east of
north). This value is consistent with a lower-significance proper motion value
derived from infrared observations of the source over the past three years,
also reported here. Given its distance of 3.5+-0.5 kpc, the implied transverse
velocity corrected to the local standard of rest is 212+-35 km/s (1 sigma). The
measured velocity is slightly below the average for normal young neutron stars,
indicating that the mechanism(s) of magnetar birth need not lead to high
neutron star velocities. We also use Australia Telescope Compact Array, Very
Large Array, and these VLBA observations to set limits on any diffuse emission
associated with the source on a variety of spatial scales, concluding that the
radio emission from XTE J1810-197 is >96% pulsed.Comment: Accepted for publication in The Astrophysical Journal. Six pages, 2
figure
Upper Critical Field in a Spin-Charge Separated Superconductor
It is demonstrated that the spatial decay of the pair propagator in a
Luttinger liquid with spin charge separation contains a logarithmic correction
relative to the free fermi gas result in a finite interval between the spin and
charge thermal lengths. It is argued that similar effects can be expected in
higher dimensional systems with spin charge separation and that the temperature
dependence of the upper critical field curve is a probe of this
effect.Comment: 3 pages, postscript file (compressed and uuencoded
Accurate sampling using Langevin dynamics
We show how to derive a simple integrator for the Langevin equation and
illustrate how it is possible to check the accuracy of the obtained
distribution on the fly, using the concept of effective energy introduced in a
recent paper [J. Chem. Phys. 126, 014101 (2007)]. Our integrator leads to
correct sampling also in the difficult high-friction limit. We also show how
these ideas can be applied in practical simulations, using a Lennard-Jones
crystal as a paradigmatic case
Intrinsic profiles and capillary waves at homopolymer interfaces: a Monte Carlo study
A popular concept which describes the structure of polymer interfaces by
``intrinsic profiles'' centered around a two dimensional surface, the ``local
interface position'', is tested by extensive Monte Carlo simulations of
interfaces between demixed homopolymer phases in symmetric binary (AB)
homopolymer blends, using the bond fluctuation model. The simulations are done
in an LxLxD geometry. The interface is forced to run parallel to the LxL planes
by imposing periodic boundary conditions in these directions and fixed boundary
conditions in the D direction, with one side favoring A and the other side
favoring B. Intrinsic profiles are calculated as a function of the ``coarse
graining length'' B by splitting the system into columns of size BxBxD and
averaging in each column over profiles relative to the local interface
position. The results are compared to predictions of the self-consistent field
theory. It is shown that the coarse graining length can be chosen such that the
interfacial width matches that of the self-consistent field profiles, and that
for this choice of B the ``intrinsic'' profiles compare well with the
theoretical predictions.Comment: to appear in Phys. Rev.
Fourth Order Algorithms for Solving the Multivariable Langevin Equation and the Kramers Equation
We develop a fourth order simulation algorithm for solving the stochastic
Langevin equation. The method consists of identifying solvable operators in the
Fokker-Planck equation, factorizing the evolution operator for small time steps
to fourth order and implementing the factorization process numerically. A key
contribution of this work is to show how certain double commutators in the
factorization process can be simulated in practice. The method is general,
applicable to the multivariable case, and systematic, with known procedures for
doing fourth order factorizations. The fourth order convergence of the
resulting algorithm allowed very large time steps to be used. In simulating the
Brownian dynamics of 121 Yukawa particles in two dimensions, the converged
result of a first order algorithm can be obtained by using time steps 50 times
as large. To further demostrate the versatility of our method, we derive two
new classes of fourth order algorithms for solving the simpler Kramers equation
without requiring the derivative of the force. The convergence of many fourth
order algorithms for solving this equation are compared.Comment: 19 pages, 2 figure
Viscoelastic Effect on Hydrodynamic Relaxation in Polymer Solutions
The viscoelastic effect on the hydrodynamic relaxation in semidilute polymer
solutions is investigated. From the linearized two-fluid model equations, we
predict that the dynamical asymmetry coupling between the velocity fluctuations
and the viscoelastic stress influences on the hydrodynamic relaxation process,
resulting in a wave-number-dependent shear viscosity.Comment: 7pages; To be published in Journal of the Physical Society of
Japan,Vol 72,No2,(2003
Interfacial layering in a three-component polymer system
We study theoretically the temporal evolution and the spatial structure of
the interface between two polymer melts involving three different species (A,
A* and B). The first melt is composed of two different polymer species A and A*
which are fairly indifferent to one another (Flory parameter chi_AA* ~ 0). The
second melt is made of a pure polymer B which is strongly attracted to species
A (chi_AB 0). We then show
that, due to these contradictory tendencies, interesting properties arise
during the evolution of the interface after the melts are put into contact: as
diffusion proceeds, the interface structures into several adjacent
"compartments", or layers, of differing chemical compositions, and in addition,
the central mixing layer grows in a very asymmetric fashion. Such unusual
behaviour might lead to interesting mechanical properties, and demonstrates on
a specific case the potential richness of multi-component polymer interfaces
(as compared to conventional two-component interfaces) for various
applications.Comment: Revised version, to appear in Macromolecule
Discovery of X-rays from the supernova remnant G0.9+0.1
During the survey of the Galactic Center region, we have
discovered X-ray emission from the central region of the supernova remnant
G0.9+0.1. The high interstellar absorption (N_H about 3 times 10^{23} cm^-2) is
consistent with a distance of order of 10 kpc and, correspondingly, an X-ray
luminosity of about 10^{35} erg s^{-1}. Although we cannot completely rule out
a thermal origin of the X-ray emission, its small angular extent (radius of
about 2'), the good fit with a power law, the presence of a flat spectrum radio
core, and the estimated SNR age of a few thousand years, favour the
interpretation in terms of synchrotron emission powered by a young, energetic
pulsar.Comment: 4 pages, 1 figure. Uses espcrc2.sty (included). To appear in The
Active X-ray Sky: Results from BeppoSAX and Rossi-XTE, Nuclear Physics B
Proceedings Supplements, L. Scarsi, H. Bradt, P. Giommi and F. Fiore (eds.),
Elsevier Science B.
The Host Galaxy of the Gamma--Ray Burst 971214
We report on Hubble Space Telescope (HST) observations of the host galaxy of
GRB 971214, about four months after the burst. The redshift of the proposed
host galaxy at z=3.418, combined with optical and radio observations of the
burst afterglow, implies the extremely large isotropic energy release from the
burst in gamma-rays of approximately 3x10^53 ergs, some two orders of magnitude
higher than the previously commonly assumed numbers. The positional offset
between the optical transient observed at the Keck telescope and the centroid
of the proposed host galaxy in the HST image is 0.14 -+0.07 arcsec. We find no
evidence in our deep HST image for a chance foreground galaxy superposed along
the line of sight to the proposed host at z=3.418. The morphology and
photometric properties of this galaxy, such as the total flux, morphology,
radial surface profile and scale length, are typical as compared to other,
spectroscopically confirmed z>3 galaxies.Comment: LaTex, 10 pages, 3 figures, accepted to ApJ
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