80 research outputs found
Comparison of binary black hole initial data sets
We present improvements to construction of binary black hole initial data
used in SpEC (the Spectral Einstein Code). We introduce new boundary conditions
for the extended conformal thin sandwich elliptic equations that enforce the
excision surfaces to be slightly inside rather than on the apparent horizons,
thus avoiding extrapolation into the black holes at the last stage of initial
data construction. We find that this improves initial data constraint
violations near and inside the apparent horizons by about 3 orders of
magnitude. We construct several initial data sets that are intended to be
astrophysically equivalent but use different free data, boundary conditions,
and initial gauge conditions. These include free data chosen as a superposition
of two black holes in time-independent horizon-penetrating harmonic and damped
harmonic coordinates. We also implement initial data for which the initial
gauge satisfies the harmonic and damped harmonic gauge conditions; this can be
done independently of the free data, since this amounts to a choice of the time
derivatives of the lapse and shift. We compare these initial data sets by
evolving them. We show that the gravitational waveforms extracted during the
evolution of these different initial data sets agree very well after excluding
initial transients. However, we do find small differences between these
waveforms, which we attribute to small differences in initial orbital
eccentricity, and in initial BH masses and spins, resulting from the different
choices of free data. Among the cases considered, we find that superposed
harmonic initial data leads to significantly smaller transients, smaller
variation in BH spins and masses during these transients, smaller constraint
violations, and more computationally efficient evolutions. Finally, we study
the impact of initial data choices on the construction of zero-eccentricity
initial data.Comment: Matches PRD version. 17 pages, 10 figure
Surrogate models for precessing binary black hole simulations with unequal masses
Only numerical relativity simulations can capture the full complexities of
binary black hole mergers. These simulations, however, are prohibitively
expensive for direct data analysis applications such as parameter estimation.
We present two new fast and accurate surrogate models for the outputs of these
simulations: the first model, NRSur7dq4, predicts the gravitational waveform
and the second model, \RemnantModel, predicts the properties of the remnant
black hole. These models extend previous 7-dimensional, non-eccentric
precessing models to higher mass ratios, and have been trained against 1528
simulations with mass ratios and spin magnitudes , with generic spin directions. The waveform model, NRSur7dq4, which begins
about 20 orbits before merger, includes all spin-weighted
spherical harmonic modes, as well as the precession frame dynamics and spin
evolution of the black holes. The final black hole model, \RemnantModel, models
the mass, spin, and recoil kick velocity of the remnant black hole. In their
training parameter range, both models are shown to be more accurate than
existing models by at least an order of magnitude, with errors comparable to
the estimated errors in the numerical relativity simulations. We also show that
the surrogate models work well even when extrapolated outside their training
parameter space range, up to mass ratios .Comment: Matches published version. Models publicly available at
https://zenodo.org/record/3455886#.XZ9s1-dKjBI and
https://pypi.org/project/surfinB
Comparison of binary black hole initial data sets
We present improvements to the construction of binary black hole initial data used in the Spectral Einstein Code (SpEC). We introduce new boundary conditions for the extended conformal thin sandwich elliptic equations that enforce the excision surfaces to be slightly inside rather than on the apparent horizons, thus avoiding extrapolation into the black holes at the last stage of initial data construction. We find that this improves initial data constraint violations near and inside the apparent horizons by about 3 orders of magnitude. We construct several initial data sets that are intended to be astrophysically equivalent but use different free data, boundary conditions, and initial gauge conditions. These include free data chosen as a superposition of two black holes in time-independent horizon-penetrating harmonic and damped harmonic coordinates. We also implement initial data for which the initial gauge satisfies the harmonic and damped harmonic gauge conditions; this can be done independently of the free data, since this amounts to a choice of the time derivatives of the lapse and shift. We compare these initial data sets by evolving them. We show that the gravitational waveforms extracted during the evolution of these different initial data sets agree very well after excluding initial transients. However, we do find small differences between these waveforms, which we attribute to small differences in initial orbital eccentricity, and in initial BH masses and spins, resulting from the different choices of free data. Among the cases considered, we find that superposed harmonic initial data lead to significantly smaller transients, smaller variation in BH spins and masses during these transients, smaller constraint violations, and more computationally efficient evolutions. Finally, we study the impact of initial data choices on the construction of zero-eccentricity initial data
Surrogate model of hybridized numerical relativity binary black hole waveforms
Numerical relativity (NR) simulations provide the most accurate binary black
hole gravitational waveforms, but are prohibitively expensive for applications
such as parameter estimation. Surrogate models of NR waveforms have been shown
to be both fast and accurate. However, NR-based surrogate models are limited by
the training waveforms' length, which is typically about 20 orbits before
merger. We remedy this by hybridizing the NR waveforms using both
post-Newtonian and effective one body waveforms for the early inspiral. We
present NRHybSur3dq8, a surrogate model for hybridized nonprecessing numerical
relativity waveforms, that is valid for the entire LIGO band (starting at
) for stellar mass binaries with total masses as low as
. We include the and spin-weighted
spherical harmonic modes but not the or modes. This model has
been trained against hybridized waveforms based on 104 NR waveforms with mass
ratios , and , where
() is the spin of the heavier (lighter) BH in the direction of
orbital angular momentum. The surrogate reproduces the hybrid waveforms
accurately, with mismatches over the mass range
. At high masses
(), where the merger and ringdown are more prominent, we
show roughly two orders of magnitude improvement over existing waveform models.
We also show that the surrogate works well even when extrapolated outside its
training parameter space range, including at spins as large as 0.998. Finally,
we show that this model accurately reproduces the spheroidal-spherical mode
mixing present in the NR ringdown signal.Comment: Matches PRD version. Model publicly available at
https://zenodo.org/record/2549618#.XJvMrutKii4. 18 pages, 12 figure
Concurrent Development and Certification of SOFTCOMAG 49AA Alloy for Aeronautical Applications
Softcomag 49AA alloy consisting of 49 wt per cent Fe, 49 wt. per cent Co and 2 wt per cent V is a soft magnetic alloy with a combination of very high saturation magnetisation and high magnetostriction and used for several aeronautical applications such as generators (stators and rotors), fixed iron moving armature units etc. Though this alloy is brittle in nature, it can be formed into hot rolled bars and cold rolled sheets by adopting suitable thermo mechanical treatments. In order to indigenise and subsequent type certification for aeronautical applications, the alloy was produced using 100 per cent virgin raw materials in a vacuum induction melting (VIM) furnace which not only ensures substantial reduction of inclusions, but also the production of homogeneous alloy as a result of induction stirring. The chemical composition was examined and hot working parameters of the alloy were so optimised that they would result in the best combination of magnetic, physical and mechanical properties for the end use, which forms the central theme behind the developmental activity that was simultaneously covered by a comprehensive certification process. The material thus produced is subjected to stringent quality control checks in accordance with stipulated airworthiness norms. The paper discusses in detail the indigenisation efforts and airworthiness certification of the alloy Softcomag 49AA and its comparison with equivalent grades, namely PERMENDUR 49 and VACOFLUX 50.Defence Science Journal, 2012, 62(1), pp.67-72, DOI:http://dx.doi.org/10.14429/dsj.62.1093
Impact of subdominant modes on the interpretation of gravitational-wave signals from heavy binary black hole systems
Over the past year, a handful of new gravitational wave models have been developed to include multiple harmonic modes thereby enabling for the first time fully Bayesian inference studies including higher modes to be performed. Using one recently developed numerical relativity surrogate model, NRHybSur3dq8, we investigate the importance of higher modes on parameter inference of coalescing massive binary black holes. We focus on examples relevant to the current three-detector network of observatories, with a detector-frame mass set to
120 M⊙ and with signal amplitude values that are consistent with plausible candidates for the next few observing runs. We show that for such systems the higher mode content will be important for interpreting coalescing binary black holes, reducing systematic bias, and computing properties of the remnant object. Even for comparable-mass binaries and at low signal amplitude, the omission of higher modes can influence posterior probability distributions. We discuss the impact of our results on source population inference and self-consistency tests of general relativity. Our work can be used to better understand asymmetric binary black hole merger events, such as GW190412. Higher modes are critical for such systems, and their omission usually produces substantial parameter biases
Eccentric binary black hole surrogate models for the gravitational waveform and remnant properties: comparable mass, nonspinning case
We develop new strategies to build numerical relativity surrogate models for
eccentric binary black hole systems, which are expected to play an increasingly
important role in current and future gravitational-wave detectors. We introduce
a new surrogate waveform model, \texttt{NRSur2dq1Ecc}, using 47 nonspinning,
equal-mass waveforms with eccentricities up to when measured at a
reference time of before merger. This is the first waveform model that
is directly trained on eccentric numerical relativity simulations and does not
require that the binary circularizes before merger. The model includes the
, , and spin-weighted spherical harmonic modes. We also
build a final black hole model, \texttt{NRSur2dq1EccRemnant}, which models the
mass, and spin of the remnant black hole. We show that our waveform model can
accurately predict numerical relativity waveforms with mismatches , while the remnant model can recover the final mass and dimensionless
spin with absolute errors smaller than and respectively. We demonstrate that the waveform model can also
recover subtle effects like mode-mixing in the ringdown signal without any
special ad-hoc modeling steps. Finally, we show that despite being trained only
on equal-mass binaries, \texttt{NRSur2dq1Ecc} can be reasonably extended up to
mass ratio with mismatches for eccentricities
smaller than as measured at a reference time of before
merger. The methods developed here should prove useful in the building of
future eccentric surrogate models over larger regions of the parameter space.Comment: 19 pages, 15 figure
- …