892 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
Massive disk formation in the tidal disruption of a neutron star by a nearly extremal black hole
Black hole-neutron star (BHNS) binaries are important sources of
gravitational waves for second-generation interferometers, and BHNS mergers are
also a proposed engine for short, hard gamma-ray bursts. The behavior of both
the spacetime (and thus the emitted gravitational waves) and the neutron star
matter in a BHNS merger depend strongly and nonlinearly on the black hole's
spin. While there is a significant possibility that astrophysical black holes
could have spins that are nearly extremal (i.e. near the theoretical maximum),
to date fully relativistic simulations of BHNS binaries have included
black-hole spins only up to =0.9, which corresponds to the black hole
having approximately half as much rotational energy as possible, given the
black hole's mass. In this paper, we present a new simulation of a BHNS binary
with a mass ratio and black-hole spin =0.97, the highest simulated
to date. We find that the black hole's large spin leads to the most massive
accretion disk and the largest tidal tail outflow of any fully relativistic
BHNS simulations to date, even exceeding the results implied by extrapolating
results from simulations with lower black-hole spin. The disk appears to be
remarkably stable. We also find that the high black-hole spin persists until
shortly before the time of merger; afterwards, both merger and accretion spin
down the black hole.Comment: 20 pages, 10 figures, submitted to Classical and Quantum Gravit
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
Gravity Waves, Chaos, and Spinning Compact Binaries
Spinning compact binaries are shown to be chaotic in the Post-Newtonian
expansion of the two body system. Chaos by definition is the extreme
sensitivity to initial conditions and a consequent inability to predict the
outcome of the evolution. As a result, the spinning pair will have
unpredictable gravitational waveforms during coalescence. This poses a
challenge to future gravity wave observatories which rely on a match between
the data and a theoretical template.Comment: Final version published in PR
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
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