1.5 Compact binaries with spinning components
1.5.2 Effective spin inspiral-merger-ringdown approximation
the post-Newtonian expansion until 1.5PN order. The spin effects in the waveform are completely known up to 2.5PN order (v5), and are described by the following parameters:
β = 113χred/12, σ0 =
−12769 (4η−81) 16 (76η−113)2
χ2red,
γ0 = 565 17136η2+ 135856η−146597 2268 (76η−113)
! χred,
(1.66)
where χred is the reduced-spin parameter. Defining the dimensionless spin ~χi = c~Si/Gm2i, the symmetric and antisymmetric combinations of the spins
χs ≡ 1
2 (~χ1+χ~2)·Lˆ (1.67)
χa ≡ 1
2 (~χ1−χ~2)·L,ˆ (1.68)
and the asymmetric mass ratioδ≡(m1−m2)/m, the reduced spin parameter is given by
χred≡χs+δχa−76η
113χs. (1.69)
One of the main advantages of this waveform family for data analysis is that it reduces the parameter space of aligned spin systems from four to three dimensions, making it appealing as a first step towards integrating spin effects into our analysis pipelines. We demonstrate in Chap. 6 that these waveforms, using only three parameters, capture greater than 90% of the search volume for generically spinning binaries withm1+m2≤12M, and furthermore using these waveforms in a pipeline could give up to a 50% increase in detection rate for binary neutron star systems4.
NINJA-2 waveform catalog 32 6
-0.5
0.0
0.5
1.0 c1
-0.5 0.0
0.5 1.0
c2 2
4 6 8 10
q
Figure 2. Mass ratio q and dimensionless spins i of the NINJA-2 hybrid waveform submissions.
M !22 is the frequency of the (`, m) = (2,±2) harmonic. In practice, hybridization fits were performed over a frequency range as summarized in Sec. 4, and the average frequency, and frequency of the average time of the fitting interval were always chosen belowM !22 0.075, with two exceptions as seen in Table 1: The nonspinning Llama waveforms at mass ratiosq= 1,2. As seen in Fig. (7) these do however show excellent overlaps with comparison waveforms.
The waveforms were submitted with the complex GW strain functionh+ ih⇥ decomposed into modes using spin-weighted spherical harmonics 2Y`m of weight s= 2. Although most of the power is in the (`, m) = (2,±2) modes, we encouraged (but did not require), the submission of additional subdominant modes. The accuracy studies in this paper focus on the (`, m) = (2,2) mode; further work is required to study the accuracy of the contributed subdominant modes. A total of 63 waveforms from 8 groups were contributed to the NINJA-2 catalog. There are 46 distinct numerical waveforms; some of these waveforms have been hybridized with multiple pN waveforms. The NINJA-2 catalog is summarized in Table 1, and a map of the parameter values is shown in Fig. 2. In the next section, we describe in more detail the numerical methods used to generate these waveforms and present additional plots in Figs. 3 and 4.
3. Numerical Methods
3.1. Summary of contributions
The NINJA-2 data set contains both hybrid and original numerical relativity waveforms, in a data format that is summarized in Sec. 3.2 below, and described in detail in Ref. [60]. The contributed waveforms cover 29 di↵erent black hole Figure 1.9: The dimensionality problem for numerical relativity simulations of compact binary mergers. We show the available numerical relativity simulations from the NINJA2 catalog [51]. Numerical relativity solutions scale in total mass, so there is only one mass parameter to cover, but there are six spin parameters.
Here we only show the aligned spin simulations, in which there are only two extra parametersχ1 andχ2. The paucity of coverage in this parameter space makes it difficult to construct even phenomenological models that interpolate between the solutions.
parameters. Covering the whole space with numerical simulations to the level that is useful for gravitational wave searches is at present impractical, as illustrated in Fig. 1.9. However, the nu- merical solutions can still provide insight into the basic behavior of systems during the merger. For the purpose of using the waveforms in gravitational wave searches, we can use these simulations as inspiration for phenomenological models that cover the parameter space continuously.
First, we have to develophybrid waveforms, which extend the short numerical relativity solution down to the early inspiral. Hybridization works by choosing a frequency interval [f1, f2], which includes the early part of the numerical simulation, and “matching” the NR waveform to a post- Newtonian one in that interval. The parameters for the post-Newtonian system need not be the same as the parameters of the numerically simulated system. Instead, a suitable post-Newtonian signal is chosen by performing a least squares analysis over the chosen frequency band, allowing the post-Newtonian source parameters to vary; this least squares fitting can be performed on phase, amplitude, or some other quantity of interest. Finally, the two waveforms are combined in the transition region [f1, f2] by a weighted sum such that atf1 the waveform exactly matches the post- Newtonian waveform, and atf2the waveform exactly matches the numerical relativity waveform.
TheIMRPhenomBmodel [19] takes a collection of such hybrid waveforms to fit a parameterized ansatz for the amplitude and phase of the complete IMR signal, motivated by both the known form of the signal in the post-Newtonian limit, the known form of the signal in the ringdown limit, and the anecdotal understanding of the waveform in the merger regime. Our primary interest in IMRPhenomBcomes from the fact that it models aligned spin effects and does so with only one spin parameter, theeffective spin, given by
χeff = m1χ1+m2χ2
m1+m2
. (1.70)
We emphasize that this parameteris not the same as theχred parameter introduced above (except in the limit that η → 0). We then introduce phenomenological parameters and an ansatz for the shape of the signal,
A(f) =Cf−7/6
1 +α2v2+α3v3 iff < f1
1 +1v+2v2 iff1≤f ≤f2
L(f, f2, σ) iff2≤f ≤f3
(1.71)
Ψ(f) = 2πf t0+φ0+ 3
128ηv5 1 + X7 k=2
ψkvk
!
. (1.72)
The three regimes correspond to the inspiral, merger, and ringdown portions of the binary evolution.
Note that the form of the amplitude and phase in thef ≤f1 (inspiral) regime are functionally the same as in the post-Newtonian expansion, though the coefficients are now free parameters for the model. The functional form for the merger is an ansatz inspired by available numerical simulations of the binary coalescence. The final stage (ringdown) of the binary evolution is modeled with a one-sided Lorenztian, which asymptotically goes asf−2. The hybrid waveforms used to develop this model were constructed from numerical relativity simulations which spanned the parameter range 1≤q≤4 and−0.85≤χeff ≤0.85.
As with theTaylorF2RedSpinfamily, theIMRPhenomBwaveform family also has the appeal of the reduced dimensionality of the parameter space, at least from the perspective of designing a search. Below we use this waveform family to implement a search pipeline for coalescing binary black holes with aligned spin, and demonstrate an analysis – in real LIGO detector noise, obtained from LIGO’s fifth science run – which improves in sensitivity to signals from spinning systems when spin effects are included in the model waveforms.
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