40 research outputs found
Properties of hypermassive neutron stars formed in mergers of spinning binaries
We present numerical simulations of binary neutron star mergers, comparing
irrotational binaries to binaries of NSs rotating aligned to the orbital
angular momentum. For the first time, we study spinning BNSs employing nuclear
physics equations of state, namely the ones of Lattimer and Swesty as well as
Shen, Horowitz, and Teige. We study mainly equal mass systems leading to a
hypermassive neutron star (HMNS), and analyze in detail its structure and
dynamics. In order to exclude gauge artifacts, we introduce a novel coordinate
system used for post-processing. The results for our equal mass models show
that the strong radial oscillations of the HMNS modulate the instantaneous
frequency of the gravitational wave (GW) signal to an extend that leads to
separate peaks in the corresponding Fourier spectrum. In particular, the high
frequency peaks which are often attributed to combination frequencies can also
be caused by the modulation of the m=2 mode frequency in the merger phase. As a
consequence for GW data analysis, the offset of the high frequency peak does
not necessarily carry information about the radial oscillation frequency.
Further, the low frequency peak in our simulations is dominated by the
contribution of the plunge and the first 1-2 bounces. The amplitude of the
radial oscillations depends on the initial NS spin, which therefore has a
complicated influence on the spectrum. Another important result is that HMNSs
can consist of a slowly rotating core with an extended, massive envelope
rotating close to Keplerian velocity, contrary to the common notion that a
rapidly rotating core is necessary to prevent a prompt collapse. Finally, our
estimates on the amount of unbound matter show a dependency on the initial NS
spin, explained by the influence of the latter on the amplitude of radial
oscillations, which in turn cause shock waves.Comment: 17 pages, 20 figures Updated to version published in PR
Differentially-rotating neutron star models with a parametrized rotation profile
We analyze the impact of the choice rotation law on equilibrium sequences of
relativistic differentially-rotating neutron stars in axisymmetry. The maximum
allowed mass for each model is strongly affected by the distribution of angular
velocity along the radial direction and by the consequent degree of
differential rotation. In order to study the wide parameter space implied by
the choice of rotation law, we introduce a functional form that generalizes the
so called "j-const. law" adopted in all previous work. Using this new rotation
law we reproduce the angular velocity profile of differentially-rotating
remnants from the coalescence of binary neutron stars in various 3-dimensional
dynamical simulations. We compute equilibrium sequences of differentially
rotating stars with a polytropic equation of state starting from the
spherically symmetric static case. By analyzing the sequences at constant
ratio, T/|W|, of rotational kinetic energy to gravitational binding energy, we
find that the parameters that best describe the binary neutron star remnants
cannot produce equilibrium configurations with values of T/|W| that exceed
0.14, the criterion for the onset of the secular instability.Comment: Submitted to A&A, 6 pages, 3 figure
High-Order Numerical-Relativity Simulations of Binary Neutron Stars
We report simulations of the inspiral and merger of binary neutron stars
performed with \texttt{WhiskyTHC}, the first of a new generation of numerical
relativity codes employing higher than second-order methods for both the
spacetime and the hydrodynamic evolution. We find that the use of higher-order
schemes improves substantially the quality of the gravitational waveforms
extracted from the simulations when compared to those computed using
traditional second-order schemes. The reduced de-phasing and the faster
convergence rate allow us to estimate the phase evolution of the gravitational
waves emitted, as well as the magnitude of finite-resolution effects, without
the need of phase- or time-alignments or rescalings of the waves, as sometimes
done in other works. Furthermore, by using an additional unpublished simulation
at very high resolution, we confirm the robustness of our high convergence
order of .Comment: Submitted for the ASTRONUM-2014 proceedings. Includes a previously
unpublished high-resolution simulatio
High-Order Fully General-Relativistic Hydrodynamics: new Approaches and Tests
We present a new approach for achieving high-order convergence in fully
general-relativistic hydrodynamic simulations. The approach is implemented in
WhiskyTHC, a new code that makes use of state-of-the-art numerical schemes and
was key in achieving, for the first time, higher than second-order convergence
in the calculation of the gravitational radiation from inspiraling binary
neutron stars Radice et al. (2013). Here, we give a detailed description of the
algorithms employed and present results obtained for a series of classical
tests involving isolated neutron stars. In addition, using the
gravitational-wave emission from the late inspiral and merger of binary neutron
stars, we make a detailed comparison between the results obtained with the new
code and those obtained when using standard second-order schemes commonly
employed for matter simulations in numerical relativity. We find that even at
moderate resolutions and for binaries with large compactness, the phase
accuracy is improved by a factor 50 or more.Comment: 34 pages, 16 figures. Version accepted on CQ
On the black hole from merging binary neutron stars: how fast can it spin?
The merger of two neutron stars will in general lead to the formation of a
torus surrounding a black hole whose rotational energy can be tapped to
potentially power a short gamma-ray burst. We have studied the merger of
equal-mass binaries with spins aligned with the orbital angular momentum to
determine the maximum spin the black hole can reach. Our initial data consists
of irrotational binaries to which we add various amounts of rotation to
increase the total angular momentum. Although the initial data violates the
constraint equations, the use of the constraint-damping CCZ4 formulation yields
evolutions with violations smaller than those with irrotational initial data
and standard formulations. Interestingly, we find that a limit of exists for the dimensionless spin and that any additional angular
momentum given to the binary ends up in the torus rather than in the black
hole, thus providing another nontrivial example supporting the cosmic
censorship hypothesis.Comment: 4 pages, 2 figures Version to appear in PRD Rapid Communication
Beyond second-order convergence in simulations of binary neutron stars in full general relativity
Despite the recent rapid progress in numerical relativity, a convergence order less than the second has so far plagued codes solving the Einstein–Euler system of equations. We report simulations of the inspiral of binary neutron stars in quasi-circular orbits computed with a new code employing high-order, high-resolution shock-capturing, finite-differencing schemes that, for the first time, go beyond the second-order barrier. In particular, without any tuning or alignment, we measure a convergence order above three both in the phase and in the amplitude of the gravitational waves. Because the new code is already able to calculate waveforms with very small phase errors at modest resolutions, we are able to obtain accurate estimates of tidal effects in the inspiral that are essentially free from the large numerical viscosity typical of lower order methods, and even for the challenging large compactness and small-deformability binary considered here. We find a remarkable agreement between our Richardson-extrapolated waveform and the one from the tidally corrected post-Newtonian (PN) Taylor-T4 model, with a de-phasing smaller than 0.4 rad during the seven orbits of the inspiral and up to the contact point. Because our results can be used reliably to assess the validity of the PN or other approximations at frequencies significantly larger than those considered so far in the literature, at these compactnesses, they seem to exclude significant tidal amplifications from next to next-to-leading-order terms in the PN expansion