700 research outputs found
Numerical simulations of neutron star-black hole binaries in the near-equal-mass regime
Simulations of neutron star-black hole (NSBH) binaries generally consider
black holes with masses in the range , where we expect to find
most stellar mass black holes. The existence of lower mass black holes,
however, cannot be theoretically ruled out. Low-mass black holes in binary
systems with a neutron star companion could mimic neutron star-neutron (NSNS)
binaries, as they power similar gravitational wave (GW) and electromagnetic
(EM) signals. To understand the differences and similarities between NSNS
mergers and low-mass NSBH mergers, numerical simulations are required. Here, we
perform a set of simulations of low-mass NSBH mergers, including systems
compatible with GW170817. Our simulations use a composition and temperature
dependent equation of state (DD2) and approximate neutrino transport, but no
magnetic fields. We find that low-mass NSBH mergers produce remnant disks
significantly less massive than previously expected, and consistent with the
post-merger outflow mass inferred from GW170817 for moderately asymmetric mass
ratio. The dynamical ejecta produced by systems compatible with GW170817 is
negligible except if the mass ratio and black hole spin are at the edge of the
allowed parameter space. That dynamical ejecta is cold, neutron-rich, and
surprisingly slow for ejecta produced during the tidal disruption of a neutron
star : . We also find that the final mass of the remnant
black hole is consistent with existing analytical predictions, while the final
spin of that black hole is noticeably larger than expected -- up to for our equal mass case
Unequal mass binary neutron star simulations with neutrino transport: Ejecta and neutrino emission
We present 12 new simulations of unequal mass neutron star mergers. The simulations are performed with the SpEC code, and utilize nuclear-theory-based equations of state and a two-moment gray neutrino transport scheme with an improved energy estimate based on evolving the number density. We model the neutron stars with the SFHo, LS220, and DD2 equations of state (EOS) and we study the neutrino and matter emission of all 12 models to search for robust trends between binary parameters and emission characteristics. We find that the total mass of the dynamical ejecta exceeds 0.01  M⊙ only for SFHo with weak dependence on the mass ratio across all models. We find that the ejecta have a broad electron fraction (Y_e) distribution (≈0.06–0.48), with mean 0.2. Y_e increases with neutrino irradiation over time, but decreases with increasing binary asymmetry. We also find that the models have ejecta with a broad asymptotic velocity distribution (≈0.05–0.7c). The average velocity lies in the range 0.2c−0.3c and decreases with binary asymmetry. Furthermore, we find that disk mass increases with binary asymmetry and stiffness of the EOS. The Y_e of the disk increases with softness of the EOS. The strongest neutrino emission occurs for the models with soft EOS. For (anti) electron neutrinos we find no significant dependence of the magnitude or angular distribution or neutrino luminosity with mass ratio. The heavier neutrino species have a luminosity dependence on mass ratio but an angular distribution which does not change with mass ratio
High-accuracy waveforms for binary black hole inspiral, merger, and ringdown
The first spectral numerical simulations of 16 orbits, merger, and ringdown
of an equal-mass non-spinning binary black hole system are presented.
Gravitational waveforms from these simulations have accumulated numerical phase
errors through ringdown of ~0.1 radian when measured from the beginning of the
simulation, and ~0.02 radian when waveforms are time and phase shifted to agree
at the peak amplitude. The waveform seen by an observer at infinity is
determined from waveforms computed at finite radii by an extrapolation process
accurate to ~0.01 radian in phase. The phase difference between this waveform
at infinity and the waveform measured at a finite radius of r=100M is about
half a radian. The ratio of final mass to initial mass is M_f/M = 0.95162 +-
0.00002, and the final black hole spin is S_f/M_f^2=0.68646 +- 0.00004.Comment: 15 pages, 11 figures; New figure added, text edited to improve
clarity, waveform made availabl
Low mass binary neutron star mergers : gravitational waves and neutrino emission
Neutron star mergers are among the most promising sources of gravitational
waves for advanced ground-based detectors. These mergers are also expected to
power bright electromagnetic signals, in the form of short gamma-ray bursts,
infrared/optical transients, and radio emission. Simulations of these mergers
with fully general relativistic codes are critical to understand the merger and
post-merger gravitational wave signals and their neutrinos and electromagnetic
counterparts. In this paper, we employ the SpEC code to simulate the merger of
low-mass neutron star binaries (two neutron stars) for a set of
three nuclear-theory based, finite temperature equations of state. We show that
the frequency peaks of the post-merger gravitational wave signal are in good
agreement with predictions obtained from simulations using a simpler treatment
of gravity. We find, however, that only the fundamental mode of the remnant is
excited for long periods of time: emission at the secondary peaks is damped on
a millisecond timescale in the simulated binaries. For such low-mass systems,
the remnant is a massive neutron star which, depending on the equation of
state, is either permanently stable or long-lived. We observe strong
excitations of l=2, m=2 modes, both in the massive neutron star and in the form
of hot, shocked tidal arms in the surrounding accretion torus. We estimate the
neutrino emission of the remnant using a neutrino leakage scheme and, in one
case, compare these results with a gray two-moment neutrino transport scheme.
We confirm the complex geometry of the neutrino emission, also observed in
previous simulations with neutrino leakage, and show explicitly the presence of
important differences in the neutrino luminosity, disk composition, and outflow
properties between the neutrino leakage and transport schemes.Comment: Accepted by PRD; 23 pages; 24 figures; 4 table
Neutron star-black hole mergers with a nuclear equation of state and neutrino cooling: Dependence in the binary parameters
We present a first exploration of the results of neutron star-black hole
mergers using black hole masses in the most likely range of
, a neutrino leakage scheme, and a modeling of the neutron
star material through a finite-temperature nuclear-theory based equation of
state. In the range of black hole spins in which the neutron star is tidally
disrupted (), we show that the merger consistently
produces large amounts of cool (), unbound,
neutron-rich material (). A comparable
amount of bound matter is initially divided between a hot disk () with typical neutrino luminosity , and a cooler tidal tail. After a short period of rapid
protonization of the disk lasting , the accretion disk cools
down under the combined effects of the fall-back of cool material from the
tail, continued accretion of the hottest material onto the black hole, and
neutrino emission. As the temperature decreases, the disk progressively becomes
more neutron-rich, with dimmer neutrino emission. This cooling process should
stop once the viscous heating in the disk (not included in our simulations)
balances the cooling. These mergers of neutron star-black hole binaries with
black hole masses and black hole spins high
enough for the neutron star to disrupt provide promising candidates for the
production of short gamma-ray bursts, of bright infrared post-merger signals
due to the radioactive decay of unbound material, and of large amounts of
r-process nuclei.Comment: 20 pages, 19 figure
Post-merger evolution of a neutron star-black hole binary with neutrino transport
We present a first simulation of the post-merger evolution of a black
hole-neutron star binary in full general relativity using an energy-integrated
general relativistic truncated moment formalism for neutrino transport. We
describe our implementation of the moment formalism and important tests of our
code, before studying the formation phase of a disk after a black hole-neutron
star merger. We use as initial data an existing general relativistic simulation
of the merger of a neutron star of 1.4 solar mass with a black hole of 7 solar
mass and dimensionless spin a/M=0.8. Comparing with a simpler leakage scheme
for the treatment of the neutrinos, we find noticeable differences in the
neutron to proton ratio in and around the disk, and in the neutrino luminosity.
We find that the electron neutrino luminosity is much lower in the transport
simulations, and that the remnant is less neutron-rich. The spatial
distribution of the neutrinos is significantly affected by relativistic
effects. Over the short timescale evolved, we do not observe purely
neutrino-driven outflows. However, a small amount of material (3e-4Msun) is
ejected in the polar region during the circularization of the disk. Most of
that material is ejected early in the formation of the disk, and is fairly
neutron rich. Through r-process nucleosynthesis, that material should produce
high-opacity lanthanides in the polar region, and could thus affect the
lightcurve of radioactively powered electromagnetic transients. We also show
that by the end of the simulation, while the bulk of the disk is neutron-rich,
its outer layers have a higher electron fraction. As that material would be the
first to be unbound by disk outflows on longer timescales, the changes in Ye
experienced during the formation of the disk could have an impact on the
nucleosynthesis outputs from neutrino-driven and viscously-driven outflows.
[Abridged]Comment: 29 pages, 25 figure
Solving Einstein's Equations With Dual Coordinate Frames
A method is introduced for solving Einstein's equations using two distinct
coordinate systems. The coordinate basis vectors associated with one system are
used to project out components of the metric and other fields, in analogy with
the way fields are projected onto an orthonormal tetrad basis. These field
components are then determined as functions of a second independent coordinate
system. The transformation to the second coordinate system can be thought of as
a mapping from the original ``inertial'' coordinate system to the computational
domain. This dual-coordinate method is used to perform stable numerical
evolutions of a black-hole spacetime using the generalized harmonic form of
Einstein's equations in coordinates that rotate with respect to the inertial
frame at infinity; such evolutions are found to be generically unstable using a
single rotating coordinate frame. The dual-coordinate method is also used here
to evolve binary black-hole spacetimes for several orbits. The great
flexibility of this method allows comoving coordinates to be adjusted with a
feedback control system that keeps the excision boundaries of the holes within
their respective apparent horizons.Comment: Updated to agree with published versio
A Numerical Relativity Waveform Surrogate Model for Generically Precessing Binary Black Hole Mergers
A generic, non-eccentric binary black hole (BBH) system emits gravitational
waves (GWs) that are completely described by 7 intrinsic parameters: the black
hole spin vectors and the ratio of their masses. Simulating a BBH coalescence
by solving Einstein's equations numerically is computationally expensive,
requiring days to months of computing resources for a single set of parameter
values. Since theoretical predictions of the GWs are often needed for many
different source parameters, a fast and accurate model is essential. We present
the first surrogate model for GWs from the coalescence of BBHs including all
dimensions of the intrinsic non-eccentric parameter space. The surrogate
model, which we call NRSur7dq2, is built from the results of numerical
relativity simulations. NRSur7dq2 covers spin magnitudes up to and mass
ratios up to , includes all modes, begins about orbits
before merger, and can be evaluated in . We find the
largest NRSur7dq2 errors to be comparable to the largest errors in the
numerical relativity simulations, and more than an order of magnitude smaller
than the errors of other waveform models. Our model, and more broadly the
methods developed here, will enable studies that would otherwise require
millions of numerical relativity waveforms, such as parameter inference and
tests of general relativity with GW observations.Comment: 10 pages, 5 figures; Added report numbe
First direct comparison of non-disrupting neutron star-black hole and binary black hole merger simulations
We present the first direct comparison of numerical simulations of neutron
star-black hole and black hole-black hole mergers in full general relativity.
We focus on a configuration with non spinning objects and within the most
likely range of mass ratio for neutron star-black hole systems (q=6). In this
region of the parameter space, the neutron star is not tidally disrupted prior
to merger, and we show that the two types of mergers appear remarkably similar.
The effect of the presence of a neutron star on the gravitational wave signal
is not only undetectable by the next generation of gravitational wave
detectors, but also too small to be measured in the numerical simulations: even
the plunge, merger and ringdown signals appear in perfect agreement for both
types of binaries. The characteristics of the post-merger remnants are equally
similar, with the masses of the final black holes agreeing within dM< 5
10^{-4}M_BH and their spins within da< 10^{-3}M_BH. The rate of periastron
advance in the mixed binary agrees with previously published binary black hole
results, and we use the inspiral waveforms to place constraints on the accuracy
of our numerical simulations independent of algorithmic choices made for each
type of binary. Overall, our results indicate that non-disrupting neutron
star-black hole mergers are exceptionally well modeled by black hole-black hole
mergers, and that given the absence of mass ejection, accretion disk formation,
or differences in the gravitational wave signals, only electromagnetic
precursors could prove the presence of a neutron star in low-spin systems of
total mass ~10Msun, at least until the advent of gravitational wave detectors
with a sensitivity comparable to that of the proposed Einstein Telescope.Comment: 13 pages, 9 figure
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