30 research outputs found
Quark Matter in Neutron Star Mergers
Binary neutron star mergers are expected to be one of the most promising
source of gravitational waves (GW) for the network of laser interferometric and
bar detectors becoming operational in the next few years. The merger wave
signal is expected to be sensitive to the interior structure of the neutron
star (NS). The structure of high density phases of matter is under current
experimental investigation in heavy-ion collisions. We investigate the
dependence of the merger process and its GW signal on the presence of quarks in
these phases by performing numerical simulations, where the smoothed particle
hydrodynamics (SPH) method and the conformally flat approximation for the
3-geometry in general relativistic gravity are implemented.Comment: 4 Pages, 4 Figures, Proc. Nuclei in the Cosmos 7, 200
The influence of quark matter at high densities on binary neutron star mergers
We consider the influence of potential quark matter existing at high densities in neutron star (NS) interiors on gravitational waves (GWs) emitted in a binary NS merger event. Two types of equations of state (EoSs) at zero temperature are used - one describing pure nuclear matter and the other nuclear matter with a phase transition to quark matter at very high densities. Binary equilibrium sequences close to the innermost stable circular orbit (ISCO) are calculated to determine the GW frequencies just before the merger. It is found that the effects of the EoSs begin to play a role when gravitational masses are larger than M∞≃ 1.5 M⊙. The difference in the GW frequency at the ISCO increases by up to ≃10 per cent for the maximum mass permitted by the EoSs. We then perform three-dimensional hydrodynamic simulations for each EoS while varying the initial mass and determine the characteristic GW frequencies of the merger remnant. The implications of the presence of quark matter show up mainly in the collapse behaviour of the merger remnant. If the collapse does not take place immediately after the merger, we find a phase difference between the two EoSs in the post-merger GW signal. We also compare the GW frequencies emitted by the remnant of the merger to values obtained from simulations using a polytropic EoS and find an imprint of the non-constant adiabatic index of our EoSs. All calculations are based on the conformally flat approximation to general relativity and the GW signal from the merger simulation is extracted up to quadrupole orde
The Influence of Quark Matter at High Densities on Binary Neutron Star Mergers
We consider the influence of potential quark matter existing at high
densities in neutron star interiors on gravitational waves (GW) emitted in a
binary neutron star merger event. Two types of equations of state (EoS) at zero
temperatures are used, one describing pure nuclear matter, the other nuclear
matter with a phase transition to quark matter at very high densities. Binary
equilibrium sequences close to the innermost stable circular orbit (ISCO) are
calculated to determine the GW frequencies just before merger. It is found that
EoS effects begin to play a role for gravitational masses larger than
. The difference in the gravitational wave frequency
at the ISCO grows to up to for the maximal allowed mass given by
the EoSs used. Then, we perform 3D hydrodynamic simulations for each EoS
varying the initial mass and determine the characteristic GW frequencies of the
merger remnants. The implications of quark matter show up mainly in a different
collapse behaviour of the merger remnant. If the collapse does not take place
immediately after merger, we find a phase difference between two EoS's in the
post-merger GW signal. We also compare the GW frequencies emitted by the merger
remnant to values from simulations using a polytropic EoS and find an imprint
of the non-constant adiabatic index of our EoSs. All calculations are based on
the conformally flat (CF) approximation to general relativity and the GW signal
from the merger simulation is extracted up to quadrupole order.Comment: 13 pages, 8 figure
Merger of black hole-neutron star binaries in full general relativity
We present our latest results for simulation for merger of black hole
(BH)-neutron star (NS) binaries in full general relativity which is performed
preparing a quasicircular state as initial condition. The BH is modeled by a
moving puncture with no spin and the NS by the -law equation of state
with and corotating velocity field as a first step. The mass of the
BH is chosen to be or , and the rest-mass
of the NS with relatively large radius of the NS
--14 km. The NS is tidally disrupted near the innermost stable
orbit but --90% of the material is swallowed into the BH and resulting
disk mass is not very large as even for small BH mass . The result indicates that the system of a BH and a massive disk
of is not formed from nonspinning BH-NS binaries irrespective
of BH mass, although a disk of mass is a possible outcome
for this relatively small BH mass range as --4. Our results
indicate that the merger of low-mass BH and NS may form a central engine of
short-gamma-ray bursts.Comment: 14 pages. To appear in a special issue of Classical and Quantum
Gravity: New Frontiers in Numerical Relativit
Equilibrium sequences of irrotational binary polytropic stars : The case of double polytropic stars
Solutions to equilibrium sequences of irrotational binary polytropic stars in
Newtonian gravity are expanded in a power of , where R and
are the orbital separation of the binary system and the radius of each
star for . For each order of , we should solve ordinary
differential equations for arbitrary polytropic indices n. We show solutions
for polytropic indices n= 0.5, 1, 1.5 and 2 up to orders. Our
semi-analytic solutions can be used to check the validity of numerical
solutions.Comment: 59 pages including 15 tables and 13 figures, revtex, accepted to
Phys. Rev.
A new numerical method for constructing quasi-equilibrium sequences of irrotational binary neutron stars in general relativity
We propose a new numerical method to compute quasi-equilibrium sequences of
general relativistic irrotational binary neutron star systems. It is a good
approximation to assume that (1) the binary star system is irrotational, i.e.
the vorticity of the flow field inside component stars vanishes everywhere
(irrotational flow), and (2) the binary star system is in quasi-equilibrium,
for an inspiraling binary neutron star system just before the coalescence as a
result of gravitational wave emission. We can introduce the velocity potential
for such an irrotational flow field, which satisfies an elliptic partial
differential equation (PDE) with a Neumann type boundary condition at the
stellar surface. For a treatment of general relativistic gravity, we use the
Wilson--Mathews formulation, which assumes conformal flatness for spatial
components of metric. In this formulation, the basic equations are expressed by
a system of elliptic PDEs. We have developed a method to solve these PDEs with
appropriate boundary conditions. The method is based on the established
prescription for computing equilibrium states of rapidly rotating axisymmetric
neutron stars or Newtonian binary systems. We have checked the reliability of
our new code by comparing our results with those of other computations
available. We have also performed several convergence tests. By using this
code, we have obtained quasi-equilibrium sequences of irrotational binary star
systems with strong gravity as models for final states of real evolution of
binary neutron star systems just before coalescence. Analysis of our
quasi-equilibrium sequences of binary star systems shows that the systems may
not suffer from dynamical instability of the orbital motion and that the
maximum density does not increase as the binary separation decreases.Comment: 20 pages, 18 figures, more results of convergence tests are added,
revised version accepted for publication in PR
Models of helically symmetric binary systems
Results from helically symmetric scalar field models and first results from a
convergent helically symmetric binary neutron star code are reported here;
these are models stationary in the rotating frame of a source with constant
angular velocity omega. In the scalar field models and the neutron star code,
helical symmetry leads to a system of mixed elliptic-hyperbolic character. The
scalar field models involve nonlinear terms that mimic nonlinear terms of the
Einstein equation. Convergence is strikingly different for different signs of
each nonlinear term; it is typically insensitive to the iterative method used;
and it improves with an outer boundary in the near zone. In the neutron star
code, one has no control on the sign of the source, and convergence has been
achieved only for an outer boundary less than approximately 1 wavelength from
the source or for a code that imposes helical symmetry only inside a near zone
of that size. The inaccuracy of helically symmetric solutions with appropriate
boundary conditions should be comparable to the inaccuracy of a waveless
formalism that neglects gravitational waves; and the (near zone) solutions we
obtain for waveless and helically symmetric BNS codes with the same boundary
conditions nearly coincide.Comment: 19 pages, 7 figures. Expanded version of article to be published in
Class. Quantum Grav. special issue on Numerical Relativit
Stationary structures of irrotational binary systems -- models for close binary systems of compact stars
We propose a new numerical method to calculate irrotational binary systems
composed of compressible gaseous stars in Newtonian gravity. Assuming
irrotationality, i.e. vanishing of the vorticity vector everywhere in the star
in the inertial frame, we can introduce the velocity potential for the flow
field. Using this velocity potential we can derive a set of basic equations for
stationary states which consist of (i) the generalized Bernoulli equation, (ii)
the Poisson equation for the Newtonian gravitational potential and (iii) the
equation for the velocity potential with the Neumann type boundary condition.
We succeeded in developing a new code to compute numerically exact solutions to
these equations for the first time. Such irrotational configurations of binary
systems are appropriate models for realistic neutron star binaries composed of
inviscid gases, just prior to coalescence of two stars caused by emission of
gravitational waves. Accuracies of our numerical solutions are so high that we
can compute reliable models for fully deformed final stationary configurations
and hence determine the inner most stable circular orbit of binary neutron star
systems under the approximations of weak gravity and inviscid limit.Comment: 32 pages, 25 bitmapped ps files, to appear in ApJ supplemen
Computation of gravitational waves from inspiraling binary neutron stars in quasiequilibrium circular orbits : Formulation and calibration
Gravitational waves from binary neutron stars in quasiequilibrium circular
orbits are computed using an approximate method which we propose in this paper.
In the first step of this method, we prepare general relativistic irrotational
binary neutron stars in a quasiequilibrium circular orbit, neglecting
gravitational waves. We adopt the so-called conformal flatness approximation
for a three-metric to obtain the quasiequilibrium states in this paper. In the
second step, we compute gravitational waves, solving linear perturbation
equations in the background spacetime of the quasiequilibrium states. Comparing
numerical results with post Newtonian waveforms and luminosity of gravitational
waves from two point masses in circular orbits, we demonstrate that this method
can produce accurate waveforms and luminosity of gravitational waves. It is
shown that the effects of tidal deformation of neutron stars and strong general
relativistic gravity modify the post Newtonian results for compact binary
neutron stars in close orbits. We indicate that the magnitude of a systematic
error in quasiequilibrium states associated with the conformal flatness
approximation is fairly large for close and compact binary neutron stars.
Several formulations for improving the accuracy of quasiequilibrium states are
proposed.Comment: 26 pages, to be published in PR
Relativistic Models for Binary Neutron Stars with Arbitrary Spins
We introduce a new numerical scheme for solving the initial value problem for
quasiequilibrium binary neutron stars allowing for arbitrary spins. The coupled
Einstein field equations and equations of relativistic hydrodynamics are solved
in the Wilson-Mathews conformal thin sandwich formalism. We construct sequences
of circular-orbit binaries of varying separation, keeping the rest mass and
circulation constant along each sequence. Solutions are presented for
configurations obeying an n=1 polytropic equation of state and spinning
parallel and antiparallel to the orbital angular momentum. We treat stars with
moderate compaction ((m/R) = 0.14) and high compaction ((m/R) = 0.19). For all
but the highest circulation sequences, the spins of the neutron stars increase
as the binary separation decreases. Our zero-circulation cases approximate
irrotational sequences, for which the spin angular frequencies of the stars
increases by 13% (11%) of the orbital frequency for (m/R) = 0.14 ((m/R) = 0.19)
by the time the innermost circular orbit is reached. In addition to leaving an
imprint on the inspiral gravitational waveform, this spin effect is measurable
in the electromagnetic signal if one of the stars is a pulsar visible from
Earth.Comment: 21 pages, 14 figures. A few explanatory sentences added and some
typos corrected. Accepted for publication in Phys. Rev.