100 research outputs found

### Frequency domain model of $f$-mode dynamic tides in gravitational waveforms from compact binary inspirals

The recent detection of gravitational waves (GWs) from the neutron star
binary inspiral GW170817 has opened a unique avenue to probe matter and
fundamental interactions in previously unexplored regimes. Extracting
information on neutron star matter from the observed GWs requires robust and
computationally efficient theoretical waveform models. We develop an
approximate frequency-domain GW phase model of a main GW signature of matter:
dynamic tides associated with the neutron stars' fundamental oscillation modes
($f$-modes). We focus on nonspinning objects on circular orbits and demonstrate
that, despite its mathematical simplicity, the new "$f$-mode tidal" (fmtidal)
model is in good agreement with the effective-one-body dynamical tides model up
to GW frequencies of $\gtrsim 1$ kHz and gives physical meaning to part of the
phenomenology captured in tidal models tuned to numerical-relativity. The
advantages of the fmtidal model are that it makes explicit the dependence of
the GW phasing on the characteristic equation-of-state parameters, i.e., tidal
deformabilities and $f$-mode frequencies; it is computationally efficient; and
it can readily be added to any frequency-domain baseline waveform. The fmtidal
model is easily amenable to future improvements and provides the means for a
first step towards independently measuring additional fundamental properties of
neutron star matter beyond the tidal deformability as well as performing novel
tests of general relativity from GW observations.Comment: 7 pages, 3 figures; matches published versio

### Spin effects on gravitational waves from inspiraling compact binaries at second post-Newtonian order

We calculate the gravitational waveform for spinning, precessing compact
binary inspirals through second post-Newtonian order in the amplitude. When
spins are collinear with the orbital angular momentum and the orbits are
quasi-circular, we further provide explicit expressions for the
gravitational-wave polarizations and the decomposition into spin-weighted
spherical-harmonic modes. Knowledge of the second post-Newtonian spin terms in
the waveform could be used to improve the physical content of analytical
templates for data analysis of compact binary inspirals and for more accurate
comparisons with numerical-relativity simulations.Comment: 15 pages, expressions available in mathematica format upon reques

### Gravitational-Wave Asteroseismology with Fundamental Modes from Compact Binary Inspirals

The first detection of gravitational waves (GWs) from the binary neutron star
(NS) inspiral GW170817 has opened a unique channel for probing the fundamental
properties of matter at supra-nuclear densities inaccessible elsewhere in the
Universe. This observation yielded the first constraints on the equation of
state (EoS) of NS matter from the GW imprint of tidal interactions. Tidal
signatures in the GW arise from the response of a matter object to the
spacetime curvature sourced by its binary companion. They crucially depend on
the EoS and are predominantly characterised by the tidal deformability
parameters $\Lambda_{\ell}$, where $\ell=2,3$ denotes the quadrupole and
octupole respectively. As the binary evolves towards merger, additional
dynamical tidal effects become important when the orbital frequency approaches
a resonance with the stars' internal oscillation modes. Among these modes, the
fundamental ($f_\ell$-)modes have the strongest tidal coupling and can give
rise to a cumulative imprint in the GW signal even if the resonance is not
fully excited. Here we present the first direct constraints on fundamental
oscillation mode frequencies for GW170817 using an inspiral GW phase model with
an explicit dependence on the $f$-mode frequency and without assuming any
relation between $f_\ell$ and $\Lambda_\ell$. We rule out anomalously small
values of $f_\ell$ and, for the larger companion, determine a lower bound on
the $f_2$-mode ($f_3$-mode) frequency of $\geq 1.39$ kHz ($\geq 1.86$ kHz) at
the 90\% credible interval (CI). We then show that networks of future GW
detectors will be able to measure $f$-mode frequencies to within tens of Hz
from the inspiral alone. Such precision astroseismology will enable novel tests
of fundamental physics and the nature of compact binaries.Comment: 8 pages, 5 figure

### Remnant baryon mass outside of the black hole after a neutron star-black hole merger

Gravitational-wave (GW) and electromagnetic (EM) signals from the merger of a
Neutron Star (NS) and a Black Hole (BH) are a highly anticipated discovery in
extreme gravity, nuclear-, and astrophysics. We develop a simple formula that
distinguishes between merger outcomes and predicts the post-merger remnant
mass, validated with 75 simulations. Our formula improves on existing results
by describing critical unexplored regimes: comparable masses and higher BH
spins. These are important to differentiate NSNS from NSBH mergers, and to
infer source physics from EM signals.Comment: 9 pages, 5 figures, 2 table

### Tidal Love numbers of neutron stars

For a variety of fully relativistic polytropic neutron star models we
calculate the star's tidal Love number k2. Most realistic equations of state
for neutron stars can be approximated as a polytrope with an effective index
n~0.5-1.0. The equilibrium stellar model is obtained by numerical integration
of the Tolman-Oppenheimer-Volkhov equations. We calculate the linear l=2 static
perturbations to the Schwarzschild spacetime following the method of Thorne and
Campolattaro. Combining the perturbed Einstein equations into a single second
order differential equation for the perturbation to the metric coefficient
g_tt, and matching the exterior solution to the asymptotic expansion of the
metric in the star's local asymptotic rest frame gives the Love number. Our
results agree well with the Newtonian results in the weak field limit. The
fully relativistic values differ from the Newtonian values by up to ~24%. The
Love number is potentially measurable in gravitational wave signals from
inspiralling binary neutron stars.Comment: corrected Eqs. (20) and (23) and entries in Table (1

### Canonical Hamiltonian for an extended test body in curved spacetime: To quadratic order in spin

We derive a Hamiltonian for an extended spinning test body in a curved
background spacetime, to quadratic order in the spin, in terms of
three-dimensional position, momentum, and spin variables having canonical
Poisson brackets. This requires a careful analysis of how changes of the spin
supplementary condition are related to shifts of the body's representative
worldline and transformations of the body's multipole moments, and we employ
bitensor calculus for a precise framing of this analysis. We apply the result
to the case of the Kerr spacetime and thereby compute an explicit canonical
Hamiltonian for the test-body limit of the spinning two-body problem in general
relativity, valid for generic orbits and spin orientations, to quadratic order
in the test spin. This fully relativistic Hamiltonian is then expanded in
post-Newtonian orders and in powers of the Kerr spin parameter, allowing
comparisons with the test-mass limits of available post-Newtonian results. Both
the fully relativistic Hamiltonian and the results of its expansion can inform
the construction of waveform models, especially effective-one-body models, for
the analysis of gravitational waves from compact binaries.Comment: RevTeX, 25 pages, 2 figures. v2: Updated to match PRD version;
further references added; some changes in presentation and notation;
typographical errors corrected, most notably in Eqs. (7.51) and (7.58

### Observing and measuring the neutron-star equation-of-state in spinning binary neutron star systems

LIGO and Virgo recently observed the first binary neutron star merger,
demonstrating that gravitational-waves offer the ability to probe how matter
behaves in one of the most extreme environments in the Universe. However, the
gravitational-wave signal emitted by an inspiraling binary neutron star system
is only weakly dependent on the equation of state and extracting this
information is challenging. Previous studies have focused mainly on binary
systems where the neutron stars are spinning slowly and the main imprint of
neutron star matter in the inspiral signal is due to tidal effects. For
binaries with non-negligible neutron-star spin the deformation of the neutron
star due to its own rotation introduces additional variations in the emitted
gravitational-wave signal. Here we explore whether highly spinning binary
neutron-star systems offer a better chance to measure the equation-of-state
than weakly spinning binary-neutron star systems. We focus on the dominant
adiabatic quadrupolar effects and consider three main questions. First, we show
that equation-of-state effects can be significant in the inspiral waveforms,
and that the spin-quadrupole effect dominates for rapidly rotating neutron
stars. Second, we show that variations in the spin-quadrupole phasing are
strongly degenerate with changes in the component masses and spins, and
neglecting these terms has a negligible impact on the number of observations
with second generation observatories. Finally, we explore the bias in the
masses and spins that would be introduced by using incorrect equation-of-state
terms. Using a novel method to rapidly evaluate an approximation of the
likelihood we show that assuming the incorrect equation-of-state when measuring
source parameters can lead to a significant bias. We also find that the ability
to measure the equation-of-state is improved when considering spinning systems.Comment: 29 pages (CQG formatting), 9 figure

### Distinguishing Boson Stars from Black Holes and Neutron Stars from Tidal Interactions in Inspiraling Binary Systems

Binary systems containing boson stars---self-gravitating configurations of a
complex scalar field--- can potentially mimic black holes or neutron stars as
gravitational-wave sources. We investigate the extent to which tidal effects in
the gravitational-wave signal can be used to discriminate between these
standard sources and boson stars. We consider spherically symmetric boson stars
within two classes of scalar self-interactions: an
effective-field-theoretically motivated quartic potential and a solitonic
potential constructed to produce very compact stars. We compute the tidal
deformability parameter characterizing the dominant tidal imprint in the
gravitational-wave signals for a large span of the parameter space of each
boson star model. We find that the tidal deformability for boson stars with a
quartic self-interaction is bounded below by $\Lambda_{\rm min}\approx 280$ and
for those with a solitonic interaction by $\Lambda_{\rm min}\approx 1.3$.
Employing a Fisher matrix analysis, we estimate the precision with which
Advanced LIGO and third-generation detectors can measure these tidal parameters
using the inspiral portion of the signal. We discuss a new strategy to improve
the distinguishability between black holes/neutrons stars and boson stars by
combining deformability measurements of each compact object in a binary system,
thereby eliminating the scaling ambiguities in each boson star model. Our
analysis shows that current-generation detectors can potentially distinguish
boson stars with quartic potentials from black holes, as well as from
neutron-star binaries if they have either a large total mass or a large mass
ratio. Discriminating solitonic boson stars from black holes using only tidal
effects during the inspiral will be difficult with Advanced LIGO, but
third-generation detectors should be able to distinguish between binary black
holes and these binary boson stars.Comment: 18 pages, 8 figures. Submitted to Physical Review

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