Impact of higher harmonics in searching for gravitational waves from nonspinning binary black holes

Abstract

Current searches for gravitational waves from coalescing binary black holes (BBH) use templates that only include the dominant harmonic. In this study we use effective-one-body multipolar waveforms calibrated to numerical-relativity simulations to quantify the effect of neglecting subdominant harmonics on the sensitivity of searches. We consider both signal-to-noise ratio (SNR) and the signal-based vetoes that are used to reweight SNR. We find that neglecting subdominant modes when searching for nonspinning BBHs with component masses 3 M ⊙ ≤ m 1 , m 2 ≤ 200 M ⊙ and total mass M < 360 M ⊙ in advanced LIGO results in a negligible reduction of the reweighted SNR at detection thresholds. Subdominant modes therefore have no effect on the detection rates predicted for advanced LIGO. Furthermore, we find that if subdominant modes are included in templates the sensitivity of the search becomes worse if we use current search priors, due to an increase in false alarm probability. Templates would need to be weighted differently than what is currently done to compensate for the increase in false alarms. If we split the template bank such that subdominant modes are only used when M ≳ 100 M ⊙ and mass ratio q ≳ 4, we find that the sensitivity does improve for these intermediate mass-ratio BBHs, but the sensitive volume associated with these systems is still small compared to equal-mass systems. Using subdominant modes is therefore unlikely to substantially increase the probability of detecting gravitational waves from nonspinning BBH signals unless there is a relatively large population of intermediate mass-ratio BBHs in the universe.

Can we measure individual black-hole spins from gravitational-wave observations?

“…Higher harmonics become more important as the mass ratio is increased [33,38,[45][46][47], and also add more structure to the waveforms. We would expect the inclusion of higher harmonics to improve the measurement of individual spins, and we once again have the possibility that there are ideal configurations (and binary orientations) that make it possible to accurately measure both spins.…”

Transformations of asymptotic gravitational-wave data

“…But both this and the time offset are "extrinsic" parameters, already dealt with in searches by simply finding the element of a discrete Fourier transform with the largest magnitude [1,59,60]. The remaining rotational degrees of freedom are described in more detail elsewhere [61][62][63]. In brief, it appears that accounting for them could provide benefits for localization and parameter estimation, but could actually be counterproductive for detection.…”

Sources of Gravitational Waves: Theory and Observations

“…What are the methods by which we might be able to improve sky localisation? For example, sub-dominant harmonics in systems with large mass ratios [422,423] and binaries with rapidly spinning components [424], could both enhance sky resolution and galaxy surveys could help target specific sky patches [425]. A proper understanding of biases in the estimation of sky position due to inaccurate waveform models or the use of galaxy catalogues is necessary.…”