8,337 research outputs found
A relativistic formalism for computation of irrotational binary stars in quasi equilibrium states
We present relativistic hydrostatic equations for obtaining irrotational
binary neutron stars in quasi equilibrium states in 3+1 formalism. Equations
derived here are different from those previously given by Bonazzola,
Gourgoulhon, and Marck, and have a simpler and more tractable form for
computation in numerical relativity. We also present hydrostatic equations for
computation of equilibrium irrotational binary stars in first post-Newtonian
order.Comment: 5 pages, corrected eqs.(2.10), (2.11) and (3.1
On the Maximum Mass of Differentially Rotating Neutron Stars
We construct relativistic equilibrium models of differentially rotating
neutron stars and show that they can support significantly more mass than their
nonrotating or uniformly rotating counterparts. We dynamically evolve such
``hypermassive'' models in full general relativity and show that there do exist
configurations which are dynamically stable against radial collapse and bar
formation. Our results suggest that the remnant of binary neutron star
coalescence may be temporarily stabilized by differential rotation, leading to
delayed collapse and a delayed gravitational wave burst.Comment: 4 pages, 2 figures, uses emulateapj.sty; to appear in ApJ Letter
Possible explanation for star-crushing effect in binary neutron star simulations
A possible explanation is suggested for the controversial star-crushing
effect seen in numerical simulations of inspiraling neutron star binaries by
Wilson, Mathews and Marronetti (WMM). An apparently incorrect definition of
momentum density in the momentum constraint equation used by WMM gives rise to
a post-1-Newtonian error in the approximation scheme. We show by means of an
analytic, post-1-Newtonian calculation that this error causes an increase of
the stars' central densities which is of the order of several percent when the
stars are separated by a few stellar radii, in agreement with what is seen in
the simulations.Comment: 4 pages, 1 figure, uses revetx macros, minor revision
Black Hole-Neutron Star Mergers: Disk Mass Predictions
Determining the final result of black hole-neutron star mergers, and in
particular the amount of matter remaining outside the black hole at late times
and its properties, has been one of the main motivations behind the numerical
simulation of these systems. Black hole-neutron star binaries are amongst the
most likely progenitors of short gamma-ray bursts --- as long as massive
(probably a few percents of a solar mass), hot accretion disks are formed
around the black hole. Whether this actually happens strongly depends on the
physical characteristics of the system, and in particular on the mass ratio,
the spin of the black hole, and the radius of the neutron star. We present here
a simple two-parameter model, fitted to existing numerical results, for the
determination of the mass remaining outside the black hole a few milliseconds
after a black hole-neutron star merger (i.e. the combined mass of the accretion
disk, the tidal tail, and the potential ejecta). This model predicts the
remnant mass within a few percents of the mass of the neutron star, at least
for remnant masses up to 20% of the neutron star mass. Results across the range
of parameters deemed to be the most likely astrophysically are presented here.
We find that, for 10 solar mass black holes, massive disks are only possible
for large neutron stars (R>12km), or quasi-extremal black hole spins (a/M>0.9).
We also use our model to discuss how the equation of state of the neutron star
affects the final remnant, and the strong influence that this can have on the
rate of short gamma-ray bursts produced by black hole-neutron star mergers.Comment: 11 pages, 7 figure
Real Space Effective Interaction and Phase Transition in the Lowest Landau Level
The transition between the stripe state and the liquid state in a high
magnetic field is studied by the density-matrix renormalization-group (DMRG)
method. Systematic analysis on the ground state of two-dimensional electrons in
the lowest Landau level shows that the transition from the stripe state to the
liquid state at v=3/8 is caused by a reduction of repulsive interaction around
r=3. The same reduction of the interaction also stabilizes the incompressible
liquid states at v=1/3 and 2/5, which shows a similarity between the two liquid
states at v=3/8 and 1/3. It is also shown that the strong short-range
interaction around r=1 in the lowest Landau level makes qualitatively different
stripe correlations compared with that in higher Landau levels.Comment: 5 pages, to appear in J. Phys. Soc. Jpn. Vol.73, No.8 (2004
Mergers of binary stars: The ultimate heavy-ion experience
The mergers of black hole-neutron star binaries are calcuated using a
pseudo-general relativistic potential that incorporates post-Newtonian corrections. Both normal matter neutron stars and
self-bound strange quark matter stars are considered as black hole partners. As
long as the neutron stars are not too massive relative to the black hole mass,
orbital decay terminates in stable mass transfer rather than an actual merger.
For a normal neutron star, mass transfer results in a widening of the orbit but
the stable transfer ends before the minimum neutron star mass is reached. For a
strange star, mass transfer does not result in an appreciable enlargement of
the orbital separation, and the stable transfer continues until the strange
star essentially disappears. These differences might be observable through
their respective gravitational wave signatures.Comment: Contribution to QM04 proceedings. Submitted to Journal of Physics
Black hole tidal problem in the Fermi normal coordinates
We derive a tidal potential for a self-gravitating fluid star orbiting Kerr
black hole along a timelike geodesic extending previous works by Fishbone and
Marck. In this paper, the tidal potential is calculated up to the third and
fourth-order terms in , where is the stellar radius and the
orbital separation, in the Fermi-normal coordinate system following the
framework developed by Manasse and Misner. The new formulation is applied for
determining the tidal disruption limit (Roche limit) of corotating Newtonian
stars in circular orbits moving on the equatorial plane of Kerr black holes. It
is demonstrated that the third and fourth-order terms quantitatively play an
important role in the Roche limit for close orbits with R/r \agt 0.1. It is
also indicated that the Roche limit of neutron stars orbiting a stellar-mass
black hole near the innermost stable circular orbit may depend sensitively on
the equation of state of the neutron star.Comment: Correct typo
General Relativistic Simulations of Jet Formation in a Rapidly Rotating Black Hole Magnetosphere
To investigate the formation mechanism of relativistic jets in active
galactic nuclei and micro-quasars, we have developed a new general relativistic
magnetohydrodynamic code in Kerr geometry. Here we report on the first
numerical simulation of jet formation in a rapidly-rotating (a=0.95) Kerr black
hole magnetosphere. We study cases in which the Keplerian accretion disk is
both co-rotating and counter-rotating with respect to the black hole rotation.
In the co-rotating disk case, our results are almost the same as those in
Schwarzschild black hole cases: a gas pressure-driven jet is formed by a shock
in the disk, and a weaker magnetically-driven jet is also generated outside the
gas pressure-driven jet. On the other hand, in the counter-rotating disk case,
a new powerful magnetically-driven jet is formed inside the gas pressure-driven
jet. The newly found magnetically-driven jet in the latter case is accelerated
by a strong magnetic field created by frame dragging in the ergosphere. Through
this process, the magnetic field extracts the energy of the black hole
rotation.Comment: Co-rotating and counter-rotating disks; 8 pages; submitted to ApJ
letter
Axisymmetric general relativistic hydrodynamics: Long-term evolution of neutron stars and stellar collapse to neutron stars and black holes
We report a new implementation for axisymmetric simulation in full general
relativity. In this implementation, the Einstein equations are solved using the
Nakamura-Shibata formulation with the so-called cartoon method to impose an
axisymmetric boundary condition, and the general relativistic hydrodynamic
equations are solved using a high-resolution shock-capturing scheme based on an
approximate Riemann solver. As tests, we performed the following simulations:
(i) long-term evolution of non-rotating and rapidly rotating neutron stars,
(ii) long-term evolution of neutron stars of a high-amplitude damping
oscillation accompanied with shock formation, (iii) collapse of unstable
neutron stars to black holes, and (iv) stellar collapses to neutron stars. The
tests (i)--(iii) were carried out with the -law equation of state, and
the test (iv) with a more realistic parametric equation of state for
high-density matter. We found that this new implementation works very well: It
is possible to perform the simulations for stable neutron stars for more than
10 dynamical time scales, to capture strong shocks formed at stellar core
collapses, and to accurately compute the mass of black holes formed after the
collapse and subsequent accretion. In conclusion, this implementation is robust
enough to apply to astrophysical problems such as stellar core collapse of
massive stars to a neutron star and black hole, phase transition of a neutron
star to a high-density star, and accretion-induced collapse of a neutron star
to a black hole. The result for the first simulation of stellar core collapse
to a neutron star started from a realistic initial condition is also presented.Comment: 28 pages, to appear in PRD 67, 0440XX (2003
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