151 research outputs found

### Topics in Gravitational-Wave Astrophysics

In this dissertation we study the applicability of different waveform models in
gravitational wave searches for comparable mass binary black holes. We determine
the domain of applicability of the computationally inexpensive closed form models,
and the same for the semi-analytic models that have been calibrated to Numerical
Relativity simulations (and are computationally more expensive). We further explore
the option of using hybrid waveforms, constructed by numerically stitching analytic
and numerical waveforms, as filters in gravitational wave detection searches. Beyond
matched-filtering, there is extensive processing of the filter output before a detection
candidate can be confirmed. We utilize recent results from Numerical Relativity to
study the ability of LIGO searches to make detections, using (recolored) detector data.
Lastly, we develop a waveform model, using recent self-force results, that captures
the complete binary coalescence process. The self-force formalism was developed in
the context of extreme mass-ratio binaries, and we successfully extend it to model
intermediate mass-ratios

### Measuring neutron star tidal deformability with Advanced LIGO: a Bayesian analysis of neutron star - black hole binary observations

The discovery of gravitational waves (GW) by Advanced LIGO has ushered us
into an era of observational GW astrophysics. Compact binaries remain the
primary target sources for LIGO, of which neutron star-black hole (NSBH)
binaries form an important subset. GWs from NSBH sources carry signatures of
(a) the tidal distortion of the neutron star by its companion black hole during
inspiral, and (b) its potential tidal disruption near merger. In this paper, we
present a Bayesian study of the measurability of neutron star tidal
deformability $\Lambda_\mathrm{NS}\propto (R/M)^{5}$ using observation(s) of
inspiral-merger GW signals from disruptive NSBH coalescences, taking into
account the crucial effect of black hole spins. First, we find that if
non-tidal templates are used to estimate source parameters for an NSBH signal,
the bias introduced in the estimation of non-tidal physical parameters will
only be significant for loud signals with signal-to-noise ratios $> 30$. For
similarly loud signals, we also find that we can begin to put interesting
constraints on $\Lambda_\mathrm{NS}$ (factor of 1-2) with individual
observations. Next, we study how a population of realistic NSBH detections will
improve our measurement of neutron star tidal deformability. For astrophysical
populations of $disruptive$ NSBH mergers, we find 20-35 events to be sufficient
to constrain $\Lambda_\mathrm{NS}$ within $\pm 25-50\%$, depending on the
chosen equation of state. In this we also assume that LIGO will detect black
holes with masses within the astrophysical $mass$-$gap$. If the mass-gap
remains preserved in NSBHs detected by LIGO, we estimate that $25\%$
$additional$ detections will furnish comparable tidal measurement accuracy. In
both cases, we find that the loudest 5-10 events to provide most of the tidal
information, thereby facilitating targeted follow-ups of NSBHs in the upcoming
LIGO-Virgo runs.Comment: 21 pages, 17 figure

### Correlations in parameter estimation of low-mass eccentric binaries: GW151226 & GW170608

The eccentricity of binary black hole mergers is predicted to be an indicator
of the history of their formation. In particular, eccentricity is a strong
signature of dynamical formation rather than formation by stellar evolution in
isolated stellar systems. It has been shown that searches for eccentric signals
with quasi-circular templates can lead to loss of SNR, and some signals could
be missed by such a pipeline. We investigate the efficacy of the existing
quasi-circular parameter estimation pipelines to determine the source
parameters of such eccentric systems. We create a set of simulated signals with
eccentricity up to 0.3 and find that as the eccentricity increases, the
recovered mass parameters are consistent with those of a binary with up to a
$\approx 10\%$ higher chirp mass and mass ratio closer to unity. We also employ
a full inspiral-merger-ringdown waveform model to perform parameter estimation
on two gravitational wave events, GW151226 and GW170608, to investigate this
bias on real data. We find that the correlation between the masses and
eccentricity persists in real data, but that there is also a correlation
between the measured eccentricity and effective spin. In particular, using a
non-spinning prior results in a spurious eccentricity measurement for GW151226.
Performing parameter estimation with an aligned spin, eccentric model, we
constrain the eccentricities of GW151226 and GW170608 to be $<0.15$ and $<0.12$
respectively

### Accurate and efficient waveforms for compact binaries on eccentric orbits

Compact binaries that emit gravitational waves in the sensitivity band of
ground-based detectors can have non-negligible eccentricities just prior to
merger, depending on the formation scenario. We develop a purely analytic,
frequency-domain model for gravitational waves emitted by compact binaries on
orbits with small eccentricity, which reduces to the quasi-circular
post-Newtonian approximant TaylorF2 at zero eccentricity and to the
post-circular approximation of Yunes et al. (2009) at small eccentricity. Our
model uses a spectral approximation to the (post-Newtonian) Kepler problem to
model the orbital phase as a function of frequency, accounting for eccentricity
effects up to ${\cal{O}}(e^8)$ at each post-Newtonian order. Our approach
accurately reproduces an alternative time-domain eccentric waveform model for
eccentricities $e\in [0, 0.4]$ and binaries with total mass less than 12 solar
masses. As an application, we evaluate the signal amplitude that eccentric
binaries produce in different networks of existing and forthcoming
gravitational waves detectors. Assuming a population of eccentric systems
containing black holes and neutron stars that is uniformly distributed in
co-moving volume, we estimate that second generation detectors like Advanced
LIGO could detect approximately 0.1-10 events per year out to redshift $z\sim
0.2$, while an array of Einstein Telescope detectors could detect hundreds of
events per year to redshift $z \sim 2.3$.Comment: 12 pages, 6 figures, 1 appendix. Submitted to Phys. Rev. D. v2:
affiliations updated, one reference corrected. Accepted to Phys. Rev.

### Accuracy and precision of gravitational-wave models of inspiraling neutron star -- black hole binaries with spin: comparison with numerical relativity in the low-frequency regime

Coalescing binaries of neutron stars (NS) and black holes (BH) are one of the
most important sources of gravitational waves for the upcoming network of
ground based detectors. Detection and extraction of astrophysical information
from gravitational-wave signals requires accurate waveform models. The
Effective-One-Body and other phenomenological models interpolate between
analytic results and $10-30$ orbit numerical relativity (NR) merger
simulations. In this paper we study the accuracy of these models using new NR
simulations that span $36-88$ orbits, with mass-ratios and black hole spins
$(q,\chi_{BH}) = (7, \pm 0.4), (7, \pm 0.6)$, and $(5, -0.9)$. We find that:
(i) the recently published SEOBNRv1 and SEOBNRv2 models of the
Effective-One-Body family disagree with each other (mismatches of a few
percent) for black hole spins $\geq 0.5$ or $\leq -0.3$, with waveform mismatch
accumulating during early inspiral; (ii) comparison with numerical waveforms
indicate that this disagreement is due to phasing errors of SEOBNRv1, with
SEOBNRv2 in good agreement with all of our simulations; (iii) Phenomenological
waveforms disagree with SEOBNRv2 over most of the NSBH binary parameter space;
(iv) comparison with NR waveforms shows that most of the model's dephasing
accumulates near the frequency interval where it switches to a phenomenological
phasing prescription; and finally (v) both SEOBNR and post-Newtonian (PN)
models are effectual for NSBH systems, but PN waveforms will give a significant
bias in parameter recovery. Our results suggest that future gravitational-wave
detection searches and parameter estimation efforts targeted at NSBH systems
with $q\lesssim 7$ and $\chi_\mathrm{BH} \approx [-0.9, +0.6]$ will benefit
from using SEOBNRv2 templates. For larger black hole spins and/or binary
mass-ratios, we recommend the models be further investigated as suitable NR
simulations become available.Comment: 20 pages, 18 figure

### Accuracy of binary black hole waveform models for aligned-spin binaries

Coalescing binary black holes are among the primary science targets for
second generation ground-based gravitational wave (GW) detectors. Reliable GW
models are central to detection of such systems and subsequent parameter
estimation. This paper performs a comprehensive analysis of the accuracy of
recent waveform models for binary black holes with aligned spins, utilizing a
new set of $84$ high-accuracy numerical relativity simulations. Our analysis
covers comparable mass binaries ($1\le m_1/m_2\le 3$), and samples
independently both black hole spins up to dimensionless spin-magnitude of $0.9$
for equal-mass binaries and $0.85$ for unequal mass binaries. Furthermore, we
focus on the high-mass regime (total mass $\gtrsim 50M_\odot$). The two most
recent waveform models considered (PhenomD and SEOBNRv2) both perform very well
for signal detection, losing less than 0.5\% of the recoverable signal-to-noise
ratio $\rho$, except that SEOBNRv2's efficiency drops mildly for both black
hole spins aligned with large magnitude. For parameter estimation, modeling
inaccuracies of SEOBNRv2 are found to be smaller than systematic uncertainties
for moderately strong GW events up to roughly $\rho\lesssim 15$. PhenomD's
modeling errors are found to be smaller than SEOBNRv2's, and are generally
irrelevant for $\rho\lesssim 20$. Both models' accuracy deteriorates with
increased mass-ratio, and when at least one black hole spin is large and
aligned. The SEOBNRv2 model shows a pronounced disagreement with the numerical
relativity simulation in the merger phase, for unequal masses and
simultaneously both black hole spins very large and aligned. Two older waveform
models (PhenomC and SEOBNRv1) are found to be distinctly less accurate than the
more recent PhenomD and SEOBNRv2 models. Finally, we quantify the bias expected
from all GW models during parameter estimation for recovery of binary's masses
and spins.Comment: 24 pages, 15 figures, minor change

### On the accuracy and precision of numerical waveforms: Effect of waveform extraction methodology

We present a new set of 95 numerical relativity simulations of non-precessing
binary black holes (BBHs). The simulations sample comprehensively both
black-hole spins up to spin magnitude of 0.9, and cover mass ratios 1 to 3. The
simulations cover on average 24 inspiral orbits, plus merger and ringdown, with
low initial orbital eccentricities $e<10^{-4}$. A subset of the simulations
extends the coverage of non-spinning BBHs up to mass ratio $q=10$.
Gravitational waveforms at asymptotic infinity are computed with two
independent techniques, extrapolation, and Cauchy characteristic extraction. An
error analysis based on noise-weighted inner products is performed. We find
that numerical truncation error, error due to gravitational wave extraction,
and errors due to the finite length of the numerical waveforms are of similar
magnitude, with gravitational wave extraction errors somewhat dominating at
noise-weighted mismatches of $\sim 3\times 10^{-4}$. This set of waveforms will
serve to validate and improve aligned-spin waveform models for gravitational
wave science.Comment: 22 pages, 9 figure

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