On the Identification of Exotic Compact Binaries with Gravitational Waves: a Phenomenological approach

TL;DR

The paper tackles the challenge of identifying exotic compact objects in gravitational-wave data without relying on exhaustively computed NR waveforms. It introduces a phenomenological framework that encodes ECO compactness into BBH-like waveforms via contact-frequency and tapering concepts, implemented in PhenomDTaper and PhenomDECO. Through BBH-template overlap studies, residual-SNR analyses, and Bayesian inference on injections and GW150914, it demonstrates that ECOs with moderate compactness can be detected with current searches and that compactness can be inferred to ~1% accuracy, enabling an ECO discrimination pathway. The results suggest a practical, iterative approach to identify and prioritize NR studies for ECOs, with GW150914 serving as a real-world validation of the method and highlighting its potential and limitations for future observations.

Abstract

Gravitational wave (GW) astronomy has been hailed as a gateway to discovering unexpected phenomena in the universe. Over the last decade there have been close to one hundred GW observations of compact-binary mergers. While these signals are largely consistent with mergers of binary black holes, binary neutron stars, or black hole-neutron star systems, some events suggest the intriguing possibility of binaries involving exotic compact objects (ECOs). Identifying and characterising an ECO merger would require accurate ECO waveform models. Using large numbers of numerical relativity simulations to develop customised models for ECO mergers akin to those used for binary black holes, would be not only computationally expensive but also challenging due to the limited understanding of the underlying physics. Alternatively, key physical imprints of the ECO on the inspiral or merger could in principle be incorporated phenomenologically into waveform models, sufficient to quantify generic properties. In this work we present a first application of this idea to assess the detectability and distinguishability of ECO mergers, and we propose a phenomenological approach that can iteratively incorporate features of ECO mergers, laying the groundwork for an effective exotic compact object identifier in compact binary coalescences. Using Bayesian parameter estimation on the data for the GW event GW150914, we find the inferred compactness to be consistent with that expected for black holes, within this framework. The efficacy of the identifier can be refined by adding information from numerical relativity simulations involving fundamental fields. Conversely, such an identifier framework can help focus future numerical relativity and modeling efforts for exotic objects.

Figures (11)

• Figure 1: Match contours shown for PhenomD_NRTidalv2 and PhenomD for a range of compactness (tidal deformabilities $\Lambda_1=\Lambda_2=\Lambda$, $y$-axis) and total mass ($x$-axis). We expect tidal effects to have minimal impact above the $\mathcal{M}=0.97$ contour, and to be important below the $\mathcal{M}=0.8$ contour.
• Figure 2: The frequency domain amplitude from PhenomD for an equal-mass BBH of $M_{\rm{tot}} = 40 M_\odot$ is shown in dashed black. Modified amplitude for ECO binaries of the same total mass with compactness $C= 0.1, 0.25, 0.33, 0.45$ are shown by lines of increasing thickness. Even at $C=0.5$ (solid black line), the BBH amplitude is not fully reproduced.
• Figure 3: Same as fig. \ref{['fig:dtapamp']} with the amplitude parameterised by compactness. We show the amplitude for four different values of the compactness parameter, going from lower to higher values represented by progressively thicker lines. We also show that for C=0.5 PhenomDECO reproduces the BBH amplitude as shown by the black dashed line.
• Figure 4: Dependence of the frequency-domain amplitude of a BBH signal on the total mass of the binary (left) for an equal mass binary, mass ratio (center) for a total mass of $40 M_\odot$ and effective spin (right) for an equal mass, $M=40 M_\odot$ binary. The amplitude of higher total mass are depicted by lines of progressively higher thickness; similarly, progressively lower mass ratio correspond to lines of progressively higher thickness. For effective spin, we show anti-aligned spin in solid black, while aligned spins are shown by dashed, dashed-dotted and dotted lines for $\chi_{\rm{eff}}= 0.3, 0.7, 0.95$ respectively. It is easy to note that both mass ratio and total mass can interfere with an earlier cut-off of inspiral; the effect of $\chi_{\rm{eff}}$ is much more nuanced and can cause subtle changes in the shape of the amplitude profile.
• Figure 5: Fitting factor contours obtained by optimizing the match between PhenomDTaper and PhenomD over the $q-M-\chi_{\rm{eff}}$ parameter space for different ECO total masses (x-axis) and mass ratio (y-axis); top left: $C=0.1$, top right: $C=0.25$, bottom left: $C=0.33$, bottom right: $C=0.45$ and colour bar on right shows the value of the fitting factors. The black contours on the top left panel enclose the region of $q-M$ space for non-spinning ECOs that would be interesting in residual tests, if such ECOs exist in nature.