1,941 research outputs found
Thermodynamical properties of a neutral vector boson gas in a constant magnetic field
The thermodynamical properties of a neutral vector boson gas in a constant
magnetic field are studied starting from the spectrum given by Proca formalism.
Bose Einstein Condensation (BEC) and magnetization are obtained, for the three
and one dimensional cases, in the limit of low temperatures. In three
dimensions the gas undergoes a phase transition to an usual BEC in which the
critical temperature depends on the magnetic field. In one dimension a diffuse
condensate appears as for the charged vector boson gas. In both cases, the
condensation is reached not only by decreasing the temperature but also by
increasing the magnetic field. In three and one dimensions self-magnetization
is possible. The anisotropy in the pressures due to axial symmetry imposed to
the system by the magnetic field is also discussed. The astrophysical
implications are commented.Comment: 17 pages, 6 figure
Anisotropic equation of state of charged and neutral vector boson gases in a constant magnetic field. Astrophysical implications
We obtain the pressures and equations of state (EoS) of charged and neutral
vector boson gases in a constant magnetic field. The axial symmetry imposed to
the system by the field splits the pressures in the parallel and perpendicular
directions along the magnetic axis, and this leads to anisotropic equations of
state. The values of pressures and energy densities are in the order of those
of Fermi gases in compact objects. This opens the possibility to the existence
of magnetized boson stars. Under certain conditions, the perpendicular pressure
might be negative imposing a bound to the stability of the star. Other
implications of negative pressures are also discussed.Comment: 5 pages, 6 figure
Exploding Bose-Einstein condensates and collapsing neutron stars driven by critical magnetic fields
The problem of a condensate of a relativistic neutral vector boson gas
constituted of particles bearing a magnetic moment is discussed. Such a vector
boson system is expected to be formed either by parallel spin-pairing of
neutrons in a sufficiently strong magnetic field, or by neutral atoms under
specific conditions of magnetic field strength and density. A strong
self-magnetization arises due to a Bose-Einstein-like condensation. Then the
system, which may resemble the superfluid said to exist in the core of neutron
stars, becomes more unstable under transverse collapse than the ordinary
fermion gas. In the nonrelativistic limit of laboratory conditions, an analogy
with the behavior of exploding Bose-Einstein condensates for critical values of
magnetic field strength and particle density; reported by several authors, is
briefly discussed.Comment: 4 pages, no figures, revtex
Detecting Triaxiality in the Galactic Dark Matter Halo through Stellar Kinematics II: Dependence on Dark Matter and Gravity Nature
Recent studies have presented evidence that the Milky Way global potential
may be nonspherical. In this case, the assembling process of the Galaxy may
have left long lasting stellar halo kinematic fossils due to the shape of the
dark matter halo, potentially originated by orbital resonances. We further
investigate such possibility, considering now potential models further away
from CDM halos, like scalar field dark matter halos, MOND, and
including several other factors that may mimic the emergence and permanence of
kinematic groups, such as, a spherical and triaxial halo with an embedded disk
potential. We find that regardless of the density profile (DM nature),
kinematic groups only appear in the presence of a triaxial halo potential. For
the case of a MOND like gravity theory no kinematic structure is present. We
conclude that the detection of these kinematic stellar groups could confirm the
predicted triaxiality of dark halos in cosmological galaxy formation scenarios.Comment: 13 pages, 7 figures. Accepted for publication in The Astrophysical
Journal, ApJ96751R
Global stability analysis of the axisymmetric wake past a spinning bullet-shaped body
We analyze the global linear stability of the axisymmetric flow around a
spinning bullet-shaped body as a function of the Reynolds number,
, and of the rotation parameter , in the ranges and . Here,
and are the free-stream and the body rotation velocities
respectively, and is the fluid kinematic viscosity. The spectrum and the
eigenfunctions obtained allow us to explain the different bifurcations from the
axisymmetric state observed in previous numerical studies. Our results reveal
that three global eigenmodes, denoted Low-Frequency (LF), Medium-Frequency (MF)
and High-Frequency (HF) modes, become unstable in different regions of the
parameter plane. We provide precise computations of the
corresponding neutral curves, that divide the plane into four
different regions: the stable axisymmetric flow prevails for small enough
values of and , while three different frozen states, where the
wake structures co-rotate with the body at different angular velocities, take
place as a consequence of the destabilization of the LF, MF and HF modes.
Several direct numerical simulations of the nonlinear state associated to the
MF mode, identified here for the first time, are also reported to complement
the linear stability results. Finally, we point out the important fact that,
since the axisymmetric base flow is -symmetric, the theory of
equivariant bifurcations implies that the weakly non-linear regimes that emerge
close to criticality must necessarily take the form of rotating-wave states.
These states, previously referred to as frozen wakes in the literature, are
thus shown to result from the base-flow symmetry.Comment: 25 pages, 16 figures, 5 tables. Accepted for publication in J. Fluid
Mec
On propagation of photons in a magnetized medium
The aim of this work is to solve the dispersion relations near the first
excitation threshold of photon propagating along the magnetic field in the
strong field limit. We have calculated the time damping of the photon in two
particular cases: the degenerate gas as well as the diluted gas limit being
both important from the Astrophysical point of view. In particular the diluted
gas limit could describe the magnethosphere of neutron stars. The solutions
have been used to obtain a finite Quantum Faraday angle in both limits. A
resonant behavior for the Faraday angle is also obtained. To reproduce the
semi-classical result for the Faraday rotation angle the weak field limit is
considered.Comment: 5 pages, 2 figure
(Self-)Magnetized Bose-Einstein Condensate stars
We study magnetic field effects on the Equations of State (EoS) and the
structure of Bose-Einstein Condensate (BEC) stars, i.e. a compact object
composed by a gas of interacting spin one bosons formed up by the pairing of
two neutrons. To include the magnetic field in the thermodynamic description,
we suppose that particle-magnetic field and particle-particle interactions are
independent. We consider two configurations for the magnetic field: one where
it constant and externally fixed, and another where it is produced by the
bosons by self-magnetization. Since the magnetic field produces the splitting
of pressures in the directions along and perpendicular to the magnetic axis,
stable configurations of self-magnetized and magnetized BEC stars are studied
using the recently found -structure equations that describe axially
symmetric objects. The magnetized BEC stars are, in general spheroidal, less
massive and smaller than the non-magnetic ones, being these effects more
relevant at low densities. For the self-magnetized BEC stars their inner
profiles of magnetic field can be computed as a function of the equatorial
radii. The values obtained for the core and surface magnetic fields are in
agreement with those typical of compact objects.Comment: 15 pages, 20 figures, 1 tabl
A non-relativistic magnetized vector boson gas at any temperature
We study the thermodynamic properties of a neutral vector boson gas in
presence of a constant magnetic field, by means of a semi-classical approach
that allows to introduce the spin in the non-relativistic spectrum of the
bosons. Bose-Einstein condensation is obtained and it turns out to depend on
all the parameters involved in the problem: temperature, particle density and
magnetic field. An spontaneous magnetization appears at low temperature as a
consequence of the condensed state. The axial symmetry imposed in the system by
the magnetic field presence, splits the pressure in two components, one along
and another perpendicular to the magnetic axis. Under certain conditions, the
perpendicular pressure becomes negative signaling that the system undergoes a
transversal magnetic collapse. The spontaneous magnetization might be useful to
model magnetic field production inside compact stars, while the negative
pressures imposes certain limits to the temperatures and densities needed
inside these objects to support a given magnetic field
2D massless QED Hall half-integer conductivity and graphene
Starting from the photon self-energy tensor in a magnetized medium, the 3D
complete antisymmetric form of the conductivity tensor is found in the static
limit of a fermion system non-invariant under fermion-antifermion exchange.
The massless relativistic 2D fermion limit in QED is derived by using the
compactification along the dimension parallel to the magnetic field. In the
static limit and at zero temperature the main features of quantum Hall effect
(QHE) are obtained: the half-integer QHE and the minimum value proportional to
for the Hall conductivity . For typical values of graphene the plateaus
of the Hall conductivity are also reproduced.Comment: 14 pages, 2 figure
Empirical bayes formulation of the elastic net and mixed-norm models: application to the eeg inverse problem
The estimation of EEG generating sources constitutes an Inverse Problem (IP)
in Neuroscience. This is an ill-posed problem, due to the non-uniqueness of the
solution, and many kinds of prior information have been used to constrain it. A
combination of smoothness (L2 norm-based) and sparseness (L1 norm-based)
constraints is a flexible approach that have been pursued by important examples
such as the Elastic Net (ENET) and mixed-norm (MXN) models. The former is used
to find solutions with a small number of smooth non-zero patches, while the
latter imposes sparseness and smoothness simultaneously along different
dimensions of the spatio-temporal matrix solutions. Both models have been
addressed within the penalized regression approach, where the regularization
parameters are selected heuristically, leading usually to non-optimal
solutions. The existing Bayesian formulation of ENET allows hyperparameter
learning, but using computationally intensive Monte Carlo/Expectation
Maximization methods. In this work we attempt to solve the EEG IP using a
Bayesian framework for models based on mixtures of L1/L2 norms penalization
functions (Laplace/Normal priors) such as ENET and MXN. We propose a Sparse
Bayesian Learning algorithm based on combining the Empirical Bayes and the
iterative coordinate descent procedures to estimate both the parameters and
hyperparameters. Using simple but realistic simulations we found that our
methods are able to recover complicated source setups more accurately and with
a more robust variable selection than the ENET and LASSO solutions using
classical algorithms. We also solve the EEG IP using data coming from a visual
attention experiment, finding more interpretable neurophysiological patterns
with our methods, as compared with other known methods such as LORETA, ENET and
LASSO FUSION using the classical regularization approach
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