470 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
Fundamental oscillation modes of neutron stars: validity of universal relations
We study the -mode frequencies and damping times of nonrotating neutron
stars (NS) in general relativity (GR) by solving the linearized perturbation
equations, with the aim to establish "universal" relations that depend only
weakly on the equations of state (EOS). Using a more comprehensive set of EOSs,
we re-examine some proposed linearizations that describe the -mode
parameters in terms of mass and radius of the neutron star (NS), and we test a
more recent proposal for expressing the -mode parameters as quadratic
functions of the effective compactness. Our extensive results for each equation
of state considered allow us to study the accuracy of each proposal. In
particular, we find that the damping time deviates quite considerably from the
proposed linearization. We introduce a new universal relation for the product
of the -mode frequency and damping time as a function of the (ordinary)
compactness, which proved to be more accurate. The relations using the
effective compactness on the other hand also fit our data accurately. Our
results show that the maximum oscillation frequency depends strongly on the
EOS, such that the measurement of a high oscillation frequency would rule out
several EOSs. Lastly, we compare the exact mode frequencies to those obtained
in the Cowling approximation, and also to results obtained with a nonlinear
evolution code, validating the implementations of the different approaches.Comment: 10 pages, 8 figures, v2: final version accepted for publication in
Phys.Rev.
Constraint damping of the conformal and covariant formulation of the Z4 system in simulations of binary neutron stars
Following previous work in vacuum spacetimes, we investigate the
constraint-damping properties in the presence of matter of the recently
developed traceless, conformal and covariant Z4 (CCZ4) formulation of the
Einstein equations. First, we evolve an isolated neutron star with an ideal gas
equation of state and subject to a constraint-violating perturbation. We
compare the evolution of the constraints using the CCZ4 and
Baumgarte-Shibata-Shapiro-Nakamura-Oohara-Kojima (BSSNOK) systems. Second, we
study the collapse of an unstable spherical star to a black hole. Finally, we
evolve binary neutron star systems over several orbits until the merger, the
formation of a black hole, and up to the ringdown. We show that the CCZ4
formulation is stable in the presence of matter and that the constraint
violations are one or more orders of magnitude smaller than for the BSSNOK
formulation. Furthermore, by comparing the CCZ4 and the BSSNOK formulations
also for neutron star binaries with large initial constraint violations, we
investigate their influence on the errors on physical quantities. We also give
a new, simple and robust prescription for the damping parameter that removes
the instabilities found when using the fully covariant version of CCZ4 in the
evolution of black holes. Overall, we find that at essentially the same
computational costs the CCZ4 formulation provides solutions that are stable and
with a considerably smaller violation of the Hamiltonian constraint than the
BSSNOK formulation. We also find that the performance of the CCZ4 formulation
is very similar to another conformal and traceless, but noncovariant
formulation of the Z4 system, i.e. the Z4c formulation.Comment: 15 pages, 11 figures; accepted for publication in Phys. Rev.
First 100 ms of a long-lived magnetized neutron star formed in a binary neutron star merger
The recent multimessenger observation of the short gamma-ray burst (SGRB) GRB
170817A together with the gravitational wave (GW) event GW170817 provides
evidence for the long-standing hypothesis associating SGRBs with binary neutron
star (BNS) mergers. The nature of the remnant object powering the SGRB, which
could have been either an accreting black hole (BH) or a long-lived magnetized
neutron star (NS), is, however, still uncertain. General relativistic
magnetohydrodynamic (GRMHD) simulations of the merger process represent a
powerful tool to unravel the jet launching mechanism, but so far most
simulations focused the attention on a BH as the central engine, while the
long-lived NS scenario remains poorly investigated. Here, we explore the latter
by performing a GRMHD BNS merger simulation extending up to ~100 ms after
merger, much longer than any previous simulation of this kind. This allows us
to (i) study the emerging structure and amplification of the magnetic field and
observe a clear saturation at magnetic energy
erg, (ii) follow the magnetically supported expansion of the outer layers of
the remnant NS and its evolution into an ellipsoidal shape without any
surrounding torus, and (iii) monitor density, magnetization, and velocity along
the axis, observing no signs of jet formation. We also argue that the
conditions at the end of the simulation disfavor later jet formation on
subsecond timescales if no BH is formed. Furthermore, we examine the rotation
profile of the remnant, the conversion of rotational energy associated with
differential rotation, the overall energy budget of the system, and the
evolution of the GW frequency spectrum. Finally, we perform an additional
simulation where we induce the collapse to a BH ~70 ms after merger, in order
to gain insights on the prospects for massive accretion tori in case of a late
collapse. We find that...Comment: 14 pages, 16 figures, matches published version in PR
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
Numerical Inside View of Hypermassive Remnant Models for GW170817
The first multimessenger observation attributed to a merging neutron star
binary provided an enormous amount of observational data. Unlocking the full
potential of this data requires a better understanding of the merger process
and the early post-merger phase, which are crucial for the later evolution that
eventually leads to observable counterparts. In this work, we perform standard
hydrodynamical numerical simulations of a system compatible with GW170817. We
focus on a single equation of state (EOS) and two mass ratios, while neglecting
magnetic fields and neutrino radiation. We then apply newly developed
postprocessing and visualization techniques to the results obtained for this
basic setting. The focus lies on understanding the three-dimensional structure
of the remnant, most notably the fluid flow pattern, and its evolution until
collapse. We investigate the evolution of mass and angular momentum
distribution up to collapse, as well as the differential rotation along and
perpendicular to the equatorial plane. For the cases that we studied, the
remnant cannot be adequately modeled as a differentially rotating axisymetric
NS. Further, the dominant aspect leading to collapse is the GW radiation and
not internal redistribution of angular momentum. We relate features of the
gravitational wave signal to the evolution of the merger remnant, and make the
waveforms publicly available. Finally, we find that the three-dimensional
vorticity field inside the disk is dominated by medium-scale perturbances and
not the orbital velocity, with potential consequences for magnetic field
amplification effects.Comment: 20 pages, 17 figure
- …