3,638 research outputs found
Neutron star masses from hydrodynamical effects in obscured sgHMXBs
A population of obscured supergiant High Mass X-ray Binaries (sgHMXBs) has
been discovered by INTEGRAL. X-ray wind tomography of IGR J17252-3616 inferred
a slow wind velocity to account for the enhanced obscuration. The main goal of
this study is to understand under which conditions high obscuration could
occur. We have used an hydrodynamical code to simulate the flow of the stellar
wind around the neutron star. A grid of simulations was used to study the
dependency of the absorbing column density and of the X-ray light-curves on the
model parameters. A comparison between the simulation results and the
observations of IGR J17252-3616 provides an estimate on these parameters. We
have constrained the wind terminal velocity to 500-600 km/s and the neutron
star mass to 1.75-2.15 solar masses. We have confirmed that the initial
hypothesis of a slow wind velocity with a moderate mass loss rate is valid. The
mass of the neutron star can be constrained by studying its impact on the
accretion flow.Comment: A&A in pres
Invariant spin coherent states and the theory of quantum antiferromagnet in a paramagnetic phase
The consistent theory of the Heisenberg quantum antiferromagnet in the
disordered phase with short range antiferromagnetic order was developed on the
basis of the path integral for the spin coherent states. We have presented the
Lagrangian of the theory in a form which is explicitly invariant under
rotations and have found natural variables in the term of which one can
construct a natural perturbation theory. The short wave spin fluctuations are
similar to the spin wave theory ones, and the long wave spin fluctuations are
governed by the nonlinear sigma model. We have also demonstrated that the short
wave spin fluctuations have to be considered accurately in the framework of the
discrete version in time of the path integral. In the framework of our approach
we have obtained the response function for the spin fluctuations for the whole
region of the frequency and the wave vector and have
calculated the free energy of the system.Comment: 7 pages, LATEX2
Recursion and Path-Integral Approaches to the Analytic Study of the Electronic Properties of
The recursion and path-integral methods are applied to analytically study the
electronic structure of a neutral molecule. We employ a tight-binding
Hamiltonian which considers both the and valence electrons of carbon.
From the recursion method, we obtain closed-form {\it analytic} expressions for
the and eigenvalues and eigenfunctions, including the highest
occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) states, and the Green's functions. We also present the local densities
of states around several ring clusters, which can be probed experimentally by
using, for instance, a scanning tunneling microscope. {}From a path-integral
method, identical results for the energy spectrum are also derived. In
addition, the local density of states on one carbon atom is obtained; from this
we can derive the degree of degeneracy of the energy levels.Comment: 19 pages, RevTex, 6 figures upon reques
Boundary effects on the scaling of the superfluid density
We study numerically the influence of the substrate (boundary conditions) on
the finite--size scaling properties of the superfluid density in
superfluid films of thickness within the XY model employing the Monte Carlo
method. Our results suggest that the jump at the
Kosterlitz--Thouless transition temperature depends on the boundary
conditions.Comment: 2 pages, 1 Latex file, 1 postscript figure, 2 style file
Oscillons and oscillating kinks in the Abelian-Higgs model
We study the classical dynamics of the Abelian Higgs model employing an
asymptotic multiscale expansion method, which uses the ratio of the Higgs to
the gauge field amplitudes as a small parameter. We derive an effective
nonlinear Schr\"{o}dinger equation for the gauge field, and a linear equation
for the scalar field containing the gauge field as a nonlinear source. This
equation is used to predict the existence of oscillons and oscillating kinks
for certain regimes of the ratio of the Higgs to the gauge field masses.
Results of numerical simulations are found to be in very good agreement with
the analytical findings, and show that the oscillons are robust, while kinks
are unstable. It is also demonstrated that oscillons emerge spontaneously as a
result of the onset of the modulational instability of plane wave solutions of
the model. Connections of the obtained solutions with the phenomenology of
superconductors is discussed.Comment: arXiv admin note: substantial text overlap with arXiv:1306.386
Momentum distributions in ^3He-^4He liquid mixtures
We present variational calculations of the one-body density matrices and
momentum distributions for ^3He-^4He mixtures in the zero temperature limit, in
the framework of the correlated basis functions theory. The ground-state wave
function contains two- and three-body correlations and the matrix elements are
computed by (Fermi)Hypernetted Chain techniques. The dependence on the ^3He
concentration (x_3) of the ^4He condensate fraction and of the
^3He pole strength (Z_F) is studied along the P=0 isobar. At low ^3He
concentration, the computed ^4He condensate fraction is not significantly
affected by the ^3He statistics. Despite of the low x_3 values, Z_F is found to
be quite smaller than that of the corresponding pure ^3He because of the strong
^3He-^4He correlations and of the overall, large total density \rho. A small
increase of along x_3 is found, which is mainly due to the decrease
of \rho respect to the pure ^4He phase.Comment: 23 pages, 7 postscript figures, Revte
Theoretical study of the dynamic structure factor of superfluid 4He
We study the dynamic structure factor of superfluid 4He
at zero temperature in the roton momentum region and beyond using
field-theoretical Green's function techniques. We start from the
Gavoret-Nozi\`{e}res two-particle propagator and introduce the concept of
quasiparticles. We treat the residual (weak) interaction between quasiparticles
as being local in coordinate space and weakly energy dependent. Our
quasiparticle model explicitly incorporates the Bose-Einstein condensate. A
complete formula for the dynamic susceptibility, which is related to , is derived. The structure factor is numerically calculated
in a self-consistent way in the special case of a momentum independent
interaction between quasiparticles. Results are compared with experiment and
other theoretical approaches.Comment: 17 pages, 16 figure
Vela X-1 as a laboratory for accretion in High-Mass X-ray Binaries
Vela X-1 is an eclipsing high mass X-ray binary (HMXB) consisting of a 283s
accreting X-ray pulsar in a close orbit of 8.964 days around the B0.5Ib
supergiant HD77581 at a distance of just 2.4 kpc. The system is considered a
prototype of wind-accreting HMXB and it has been used as a baseline in
different theoretical or modelling studies.
We discuss the observational properties of the system and the use of the
observational data as laboratory to test recent developments in modelling the
accretion process in High-Mass X-ray Binaries (e.g., Sander et al. 2018; El
Mellah et al. 2018), which range from detailed descriptions of the wind
acceleration to modelling of the structure of the flow of matter close to the
neutron star and its variations.Comment: 4 pages, 2 figures, proceedings of the 12th INTEGRAL conference
"INTEGRAL looks AHEAD to Multimessenger astronomy" in Geneva (Switzerland) on
11-15 February 201
Ground state properties of antiferromagnetic Heisenberg spin rings
Exact ground state properties of antiferromagnetic Heisenberg spin rings with
isotropic next neighbour interaction are presented for various numbers of spin
sites and spin quantum numbers. Earlier work by Peierls, Marshall, Lieb,
Schultz and Mattis focused on bipartite lattices and is not applicable to rings
with an odd number of spins. With the help of exact diagonalization methods we
find a more general systematic behaviour which for instance relates the number
of spin sites and the individual spin quantum numbers to the degeneracy of the
ground state. These numerical findings all comply with rigorous proofs in the
cases where a general analysis could be carried out. Therefore it can be
plausibly conjectured that the ascertained properties hold for ground states of
arbitrary antiferromagnetic Heisenberg spin rings.Comment: 13 pages, 5 figures, uses epsfig.sty, submitted to Phys. Rev. B. More
information at http://www.physik.uni-osnabrueck.de/makrosysteme
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