577 research outputs found
A Two-Dimensional Hydrostatically Equilibrium Atmosphere of a Neutron Star with Given Differential Rotation
An analytic solution has been found in the Roche approximation for the
axially symmetric structure of a hydrostatically equilibrium atmosphere of a
neutron star produced by collapse. A hydrodynamic (quasione-dimensional) model
for the collapse of a rotating iron core in a massive star gives rise to a
heterogeneous rotating protoneutron star with an extended atmosphere composed
of matter from the outer part of the iron core with differential rotation
(Imshennik and Nadyozhin, 1992). The equation of state of a completely
degenerate iron gas with an arbitrary degree of relativity is taken for the
atmospheric matter. We construct a family of toroidal model atmospheres with
total masses and total angular momenta , which are acceptable for the
outer part of the collapsed iron core, in accordance with the hydrodynamic
model, as a function of constant parameters of the
specified differential rotation law in spherical
coordinates. The assumed rotation law is also qualitatively consistent with the
hydrodynamic model for the collapse of an iron core.Comment: 9 pages, 6 figures, 1 tabl
Feasibility of study magnetic proximity effects in bilayer "superconductor/ferromagnet" using waveguide-enhanced Polarized Neutron Reflectometry
A resonant enhancement of the neutron standing waves is proposed to use in
order to increase the magnetic neutron scattering from a
"superconductor/ferromagnet"(S/F) bilayer. The model calculations show that
usage of this effect allows to increase the magnetic scattering intensity by
factor of hundreds. Aspects related to the growth procedure (order of
deposition, roughness of the layers etc) as well as experimental conditions
(resolution, polarization of the neutron beam, background etc) are also
discussed.
Collected experimental data for the S/F heterostructure
Cu(32nm)/V(40nm)/Fe(1nm)/MgO confirmed the presence of a resonant 60-fold
amplification of the magnetic scattering.Comment: The manuscript of the article submitted to Crysstalography Reports.
23 pages, 5 figure
GRBs and the thermalization process of electron-positron plasmas
We discuss the temporal evolution of the pair plasma created in Gamma-Ray
Burst sources. A particular attention is paid to the relaxation of the plasma
into thermal equilibrium. We also discuss the connection between the dynamics
of expansion and the spatial geometry of the plasma. The role of the baryonic
loading parameter is emphasized.Comment: 4 pages, 3 figures, in the Proceedings of the "Gamma Ray Bursts 2007"
meeting, November 5-9, 2007, Santa Fe, New Mexico, US
On the feasibility to study inverse proximity effect in a single S/F bilayer by Polarized Neutron Reflectometry
Here we report on a feasibility study aiming to explore the potential of
Polarized Neutron Reflectometry (PNR) for detecting the inverse proximity
effect in a single superconducting/ferromagnetic bilayer. Experiments,
conducted on the V(40nm)/Fe(1nm) S/F bilayer, have shown that experimental spin
asymmetry measured at T = 0.5TC is shifted towards higher Q values compared to
the curve measured at T = 1.5TC. Such a shift can be described by the
appearance in superconducting vanadium of magnetic sub-layer with thickness of
7 nm and magnetization of +0.8 kG.Comment: Changes in the 2nd version: small mistypes are corrected. Manuscript
submitted to JETP let. 4 pages, 2 figure
A hydrodynamic model for asymmetric explosions of rapidly rotating collapsing supernovae with a toroidal atmosphere
We numerically solved the two-dimensional axisymmetric hydrodynamic problem
of the explosion of a low-mass neutron star in a circular orbit. In the initial
conditions, we assumed a nonuniform density distribution in the space
surrounding the collapsed iron core in the form of a stationary toroidal
atmosphere that was previously predicted analytically and computed numerically.
The configuration of the exploded neutron star itself was modeled by a
torus with a circular cross section whose central line almost coincided with
its circular orbit. Using an equation of state for the stellar matter and the
toroidal atmosphere in which the nuclear statistical equilibrium conditions
were satisfied, we performed a series of numerical calculations that showed the
propagation of a strong divergent shock wave with a total energy of 0.2x10^51
erg at initial explosion energy release of 1.0x10^51 erg. In our calculations,
we rigorously took into account the gravitational interaction, including the
attraction from a higher-mass (1.9M_solar) neutron star located at the
coordinate origin, in accordance with the rotational explosion mechanism for
collapsing supernovae.W e compared in detail our results with previous similar
results of asymmetric supernova explosion simulations and concluded that we
found a lower limit for the total explosion energy.Comment: 13 pages, 5 figures, 2 table
Pair plasma relaxation time scales
By numerically solving the relativistic Boltzmann equations, we compute the
time scale for relaxation to thermal equilibrium for an optically thick
electron-positron plasma with baryon loading. We focus on the time scales of
electromagnetic interactions. The collisional integrals are obtained directly
from the corresponding QED matrix elements. Thermalization time scales are
computed for a wide range of values of both the total energy density (over 10
orders of magnitude) and of the baryonic loading parameter (over 6 orders of
magnitude). This also allows us to study such interesting limiting cases as the
almost purely electron-positron plasma or electron-proton plasma as well as
intermediate cases. These results appear to be important both for laboratory
experiments aimed at generating optically thick pair plasmas as well as for
astrophysical models in which electron-positron pair plasmas play a relevant
role.Comment: Phys. Rev. E, in pres
Magnetic and Superconducting Phase Diagram of Nb/Gd/Nb trilayers
We report on a study of the structural, magnetic and superconducting
properties of Nb(25nm)/Gd()/Nb(25nm) hybrid structures of a
superconductor/ ferromagnet (S/F) type. The structural characterization of the
samples, including careful determination of the layer thickness, was performed
using neutron and X-ray scattering with the aid of depth sensitive
mass-spectrometry. The magnetization of the samples was determined by SQUID
magnetometry and polarized neutron reflectometry and the presence of magnetic
ordering for all samples down to the thinnest Gd(0.8nm) layer was shown. The
analysis of the neutron spin asymmetry allowed us to prove the absence of
magnetically dead layers in junctions with Gd interlayer thickness larger than
one monolayer. The measured dependence of the superconducting transition
temperature has a damped oscillatory behavior with well defined
positions of the minimum at =3nm and the following maximum at =4nm;
the behavior, which is in qualitative agreement with the prior work (J.S. Jiang
et al, PRB 54, 6119). The analysis of the dependence based on Usadel
equations showed that the observed minimum at =3nm can be described by the
so called "" to "" phase transition of highly transparent S/F
interfaces with the superconducting correlation length nm in
Gd. This penetration length is several times higher than for strong
ferromagnets like Fe, Co or Ni, simplifying thus preparation of S/F structures
with which are of topical interest in superconducting
spintronics
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