304 research outputs found

### Possible evolutionary transition from rapidly rotating neutron stars to strange stars due to spin-down

We present a scenario of formation of strange stars due to spin-down of {\it
rapidly rotating} neutron stars left after supernova explosions . By assuming a
process where the total baryon mass is conserved but the angular momentum is
lost due to emission of gravitational waves and/or the magnetic braking, we
find that the transition from rapidly rotating neutron stars to slowly rotating
strange stars is possible; a large amount of energy $\sim 10^{53} ergs$ could
be released. The liberated energy might become a new energy source for a
delayed explosion of supernova. Furthermore, our scenario suggests that the
supernova associated with gamma-ray bursts could become candidates for targets
in the future observation of gravitational waves.Comment: 11 pages, 3 figures, Received November 5, 200

### Hyperon mixing and universal many-body repulsion in neutron stars

A multi-pomeron exchange potential (MPP) is proposed as a model for the
universal many-body repulsion in baryonic systems on the basis of the Extended
Soft Core (ESC) bryon-baryon interaction. The strength of MPP is determined by
analyzing the nucleus-nucleus scattering with the G-matrix folding model. The
interaction in $\Lambda N$ channels is shown to reproduce well the experimental
$\Lambda$ binding energies. The equation of state (EoS) in neutron matter with
hyperon mixing is obtained including the MPP contribution, and mass-radius
relations of neutron stars are derived. It is shown that the maximum mass can
be larger than the observed one $2M_{\odot}$ even in the case of including
hyperon mixing on the basis of model-parameters determined by terrestrial
experiments

### Structure of the hadron-quark mixed phase in protoneutron stars

We study the hadron-quark phase transition in the interior of hot
protoneutron stars, combining the Brueckner-Hartree-Fock approach for hadronic
matter with the MIT bag model or the Dyson-Schwinger model for quark matter. We
examine the structure of the mixed phase constructed according to different
prescriptions for the phase transition, and the resulting consequences for
stellar properties. We find important effects for the internal composition, but
only very small influence on the global stellar properties.Comment: 6 pages, 4 figure

### Finite-size effects at the hadron-quark transition and heavy hybrid stars

We study the role of finite-size effects at the hadron-quark phase transition
in a new hybrid equation of state constructed from an ab-initio
Br\"uckner-Hartree-Fock equation of state with the realistic Bonn-B potential
for the hadronic phase and a covariant non-local Nambu--Jona-Lasinio model for
the quark phase. We construct static hybrid star sequences and find that our
model can support stable hybrid stars with an onset of quark matter below $2
M_\odot$ and a maximum mass above $2.17 M_\odot$ in agreement with recent
observations. If the finite-size effects are taken into account the core is
composed of pure quark matter. Provided that the quark vector channel
interaction is small, and the finite size effects are taken into account, quark
matter appears at densities 2-3 times the nuclear saturation density. In that
case the proton fraction in the hadronic phase remains below the value required
by the onset of the direct URCA process, so that the early onset of quark
matter shall affect on the rapid cooling of the star.Comment: version to match the one published in PR

### Neutron-star radii based on realistic nuclear interactions

The existence of neutron stars with $2M_\odot$ requires the strong stiffness
of the equation of state (EoS) of neutron-star matter. We introduce a
multi-pomeron exchange potential (MPP) working universally among 3- and
4-baryons to stiffen the EoS. Its strength is restricted by analyzing the
nucleus-nucleus scattering with the G-matrix folding model. The EoSs are
derived using the Brueckner-Hartree-Fock (BHF) and the cluster variational
method (CVM) with the nuclear interactions ESC and AV18. The mass-radius
relations are derived by solving the Tolmann-Oppenheimer-Volkoff (TOV)
equation, where the maximum masses over $2M_\odot$ are obtained on the basis of
the terrestrial data. Neutron-star radii $R$ at a typical mass $1.5M_\odot$ are
predicted to be $12.3\!\sim\!13.0$ km. The uncertainty of calculated radii is
mainly from the ratio of 3- and 4-pomeron coupling constants, which cannot be
fixed by any terrestrial experiment. Though values of $R(1.5M_\odot)$ are not
influenced by hyperon-mixing effects, finely-observed values for them indicate
degrees of EoS softening by hyperon mixing in the region of
$M\!\sim\!2M_\odot$. If $R(1.5M_\odot)$ is less than about 12.4 km, the
softening of EoS by hyperon mixing has to be weak. Useful information can be
expected by the space mission NICER offering precise measurements for
neutron-star radii within $\pm 5\%$.Comment: 8 pages, 7 figure

### Bandit Online Optimization Over the Permutahedron

The permutahedron is the convex polytope with vertex set consisting of the
vectors $(\pi(1),\dots, \pi(n))$ for all permutations (bijections) $\pi$ over
$\{1,\dots, n\}$. We study a bandit game in which, at each step $t$, an
adversary chooses a hidden weight weight vector $s_t$, a player chooses a
vertex $\pi_t$ of the permutahedron and suffers an observed loss of
$\sum_{i=1}^n \pi(i) s_t(i)$.
A previous algorithm CombBand of Cesa-Bianchi et al (2009) guarantees a
regret of $O(n\sqrt{T \log n})$ for a time horizon of $T$. Unfortunately,
CombBand requires at each step an $n$-by-$n$ matrix permanent approximation to
within improved accuracy as $T$ grows, resulting in a total running time that
is super linear in $T$, making it impractical for large time horizons.
We provide an algorithm of regret $O(n^{3/2}\sqrt{T})$ with total time
complexity $O(n^3T)$. The ideas are a combination of CombBand and a recent
algorithm by Ailon (2013) for online optimization over the permutahedron in the
full information setting. The technical core is a bound on the variance of the
Plackett-Luce noisy sorting process's "pseudo loss". The bound is obtained by
establishing positive semi-definiteness of a family of 3-by-3 matrices
generated from rational functions of exponentials of 3 parameters

### Neutrino Emission from Magnetized Proto-Neutron Stars in Relativistic Mean Field Theory

We make a perturbative calculation of neutrino scattering and absorption in
hot and dense hyperonic neutron-star matter in the presence of a strong
magnetic field. We find that the absorption cross-sections show a remarkable
angular dependence in that the neutrino absorption strength is reduced in a
direction parallel to the magnetic field and enhanced in the opposite
direction. This asymmetry in the neutrino absorbtion can be as much as 2.2 % of
the entire neutrino momentum for an interior magnetic field of \sim 2 x 10^{17}
G. We estimate the pulsar kick velocities associated with this asymmetry in a
fully relativistic mean-field theory formulation. We show that the kick
velocities calculated here are comparable to observed pulsar velocities.Comment: arXiv admin note: substantial text overlap with arXiv:1009.097

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