23,780 research outputs found
Numerical Models of Spin-Orbital Coupling in Neutron Star Binaries
We present a new numerical scheme for solving the initial value problem for
quasiequilibrium binary neutron stars allowing for arbitrary spins. We
construct sequences of circular-orbit binaries of varying separation, keeping
the rest mass and circulation constant along each sequence. The spin angular
frequency of the stars is shown to vary along the sequence, a result that can
be derived analytically in the PPN limit. This spin effect, in addition to
leaving an imprint on the gravitational waveform emitted during binary
inspiral, is measurable in the electromagnetic signal if one of the stars is a
pulsar visible from Earth.Comment: 4 pages, 3 figures. Submitted to the Proceedings of the "X Marcel
Grossmann Meeting on General Relativity" in Rio de Janeiro, Brazil, July
20-26 (2003
Critical Temperature for -Particle Condensation within a Momentum Projected Mean Field Approach
Alpha-particle (quartet) condensation in homogeneous spin-isospin symmetric
nuclear matter is investigated. The usual Thouless criterion for the critical
temperature is extended to the quartet case. The in-medium four-body problem is
strongly simplified by the use of a momentum projected mean field ansatz for
the quartet. The self-consistent single particle wave functions are shown and
discussed for various values of the density at the critical temperature
Dark energy: a quantum fossil from the inflationary Universe?
The discovery of dark energy (DE) as the physical cause for the accelerated
expansion of the Universe is the most remarkable experimental finding of modern
cosmology. However, it leads to insurmountable theoretical difficulties from
the point of view of fundamental physics. Inflation, on the other hand,
constitutes another crucial ingredient, which seems necessary to solve other
cosmological conundrums and provides the primeval quantum seeds for structure
formation. One may wonder if there is any deep relationship between these two
paradigms. In this work, we suggest that the existence of the DE in the present
Universe could be linked to the quantum field theoretical mechanism that may
have triggered primordial inflation in the early Universe. This mechanism,
based on quantum conformal symmetry, induces a logarithmic,
asymptotically-free, running of the gravitational coupling. If this evolution
persists in the present Universe, and if matter is conserved, the general
covariance of Einstein's equations demands the existence of dynamical DE in the
form of a running cosmological term whose variation follows a power law of the
redshift.Comment: LaTeX, 14 pages, extended discussion. References added. Accepted in
J. Phys. A: Mathematical and Theoretica
Bulk Viscosity in Neutron Stars from Hyperons
The contribution from hyperons to the bulk viscosity of neutron star matter
is calculated. Compared to previous works we use for the weak interaction the
one-pion exchange model rather than a current-current interaction, and include
the neutral current process. Also the sensitivity
to details of the equation of state is examined. Compared to previous works we
find that the contribution from hyperons to the bulk viscosity is about two
orders of magnitude smaller.Comment: 18 pages, to appear in Physical Review
Magnetic fields generated by r-modes in accreting millisecond pulsars
In millisecond pulsars the existence of the Coriolis force allows the
development of the so-called Rossby oscillations (r-modes) which are know to be
unstable to emission of gravitational waves. These instabilities are mainly
damped by the viscosity of the star or by the existence of a strong magnetic
field. A fraction of the observed millisecond pulsars are known to be inside
Low Mass X-ray Binaries (LMXBs), systems in which a neutron star (or a black
hole) is accreting from a donor whose mass is smaller than 1 . Here we
show that the r-mode instabilities can generate strong toroidal magnetic fields
by inducing differential rotation. In this way we also provide an alternative
scenario for the origin of the magnetars.Comment: 6 pages, 3 figures, Proceedings conference "Theoretical Nuclear
Physics", Cortona October 200
Head-on collisions of binary white dwarf--neutron stars: Simulations in full general relativity
We simulate head-on collisions from rest at large separation of binary white
dwarf -- neutron stars (WDNSs) in full general relativity. Our study serves as
a prelude to our analysis of the circular binary WDNS problem. We focus on
compact binaries whose total mass exceeds the maximum mass that a cold
degenerate star can support, and our goal is to determine the fate of such
systems. A fully general relativistic hydrodynamic computation of a realistic
WDNS head-on collision is prohibitive due to the large range of dynamical time
scales and length scales involved. For this reason, we construct an equation of
state (EOS) which captures the main physical features of NSs while, at the same
time, scales down the size of WDs. We call these scaled-down WD models
"pseudo-WDs (pWDs)". Using pWDs, we can study these systems via a sequence of
simulations where the size of the pWD gradually increases toward the realistic
case. We perform two sets of simulations; One set studies the effects of the NS
mass on the final outcome, when the pWD is kept fixed. The other set studies
the effect of the pWD compaction on the final outcome, when the pWD mass and
the NS are kept fixed. All simulations show that 14%-18% of the initial total
rest mass escapes to infinity. All remnant masses still exceed the maximum rest
mass that our cold EOS can support (1.92 solar masses), but no case leads to
prompt collapse to a black hole. This outcome arises because the final
configurations are hot. All cases settle into spherical, quasiequilibrium
configurations consisting of a cold NS core surrounded by a hot mantle,
resembling Thorne-Zytkow objects. Extrapolating our results to realistic WD
compactions, we predict that the likely outcome of a head-on collision of a
realistic, massive WDNS system will be the formation of a quasiequilibrium
Thorne-Zytkow-like object.Comment: 24 pages, 14 figures, matches PRD published version, tests of HRSC
schemes with piecewise polytropes adde
Accretion disks around binary black holes of unequal mass: GRMHD simulations near decoupling
We report on simulations in general relativity of magnetized disks onto black
hole binaries. We vary the binary mass ratio from 1:1 to 1:10 and evolve the
systems when they orbit near the binary-disk decoupling radius. We compare
(surface) density profiles, accretion rates (relative to a single, non-spinning
black hole), variability, effective -stress levels and luminosities as
functions of the mass ratio. We treat the disks in two limiting regimes: rapid
radiative cooling and no radiative cooling. The magnetic field lines clearly
reveal jets emerging from both black hole horizons and merging into one common
jet at large distances. The magnetic fields give rise to much stronger shock
heating than the pure hydrodynamic flows, completely alter the disk structure,
and boost accretion rates and luminosities. Accretion streams near the horizons
are among the densest structures; in fact, the 1:10 no-cooling evolution
results in a refilling of the cavity. The typical effective temperature in the
bulk of the disk is yielding characteristic thermal frequencies . These systems are
thus promising targets for many extragalactic optical surveys, such as LSST,
WFIRST, and PanSTARRS.Comment: 29 pages, 23 captioned figures, 3 tables, submitted to PR
On the Running of the Cosmological Constant in Quantum General Relativity
We present arguments that show what the running of the cosmological constant
means when quantum general relativity is formulated following the prescription
developed by Feynman.Comment: 5 page
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