Impact of eccentricity on the gravitational-wave searches for binary black holes: High mass case

The possible formation of stellar-mass binary black holes through dynamical interactions in dense stellar environments predicts the existence of binaries with non-negligible eccentricity in the frequency band of ground-based gravitational-wave detectors; the detection of binary black hole mergers with measurable orbital eccentricity would validate the existence of this formation channel. Waveform templates currently used in the matched-filter gravitational-wave searches of LIGO-Virgo data neglect effects of eccentricity which is expected to reduce their efficiency to detect eccentric binary black holes. Meanwhile, the sensitivity of coherent unmodeled gravitational-wave searches (with minimal assumptions about the signal model) has been shown to be largely unaffected by the presence of even sizable orbital eccentricity. In this paper, we compare the performance of two state-of-the-art search algorithms recently used by LIGO and Virgo to search for binary black holes in the second observing run (O2), quantifying their search sensitivity by injecting numerical relativity simulations of inspiral-merger-ringdown eccentric waveforms into O2 LIGO data. Our results show that the matched-filter search pycbc performs better than the unmodeled search coherent WaveBurst for the high chirp mass ($>20M_{\odot }$) and low eccentricity region ($e_{30\mathrm{Hz}}<0.3$) of parameter space. For moderate eccentricities and low chirp mass, on the other hand, the unmodeled search is more sensitive than the modeled search.

Figure 1

Top panel: time domain evolution of the eccentricity estimated from the eccentric NR simulation with ID 17 in Table 1. Mid panel: time domain 22-mode GW frequencies of the eccentric case with ID 17 from Table 1 and of the quasicircular IMRPhenomTP waveform model, highlighted in blue and green colors, respectively. Bottom panel: eccentricity as a function of the gravitational frequency of the (2,2) mode for the same configuration as in the upper panel. With vertical lines in the top and bottom plots, we have highlighted the MECO time and frequency, respectively.

Figure 2

Temporal evolution of the GW polarization state $h_{\times }(t)$ for nonspinning, eccentric stellar-mass BBHs with total mass $M_T=50M_{\odot }$ and mass ratio $q=2$, provided by numerical-relativity simulations. The characteristic orbital eccentricity of the system—defined at a reference frequency of 30 Hz—is estimated to be 0.05 (in blue, simulation ID 11) and 0.23 (in orange, simulation ID 14), respectively.

Figure 3

Top row: the left and right panels show the sensitivity range for the cWB and pycbc search pipelines for various chirp mass bins as a function of eccentricity defined at 30 Hz at an $\mathrm{IFAR}>10\mathrm{years}$, respectively. Bottom row: the left and right panels display the sensitivity range for the cWB and pycbc search pipeline for various chirp mass bins as a function of eccentricity defined at 30 Hz at an $\mathrm{IFAR}>100\mathrm{years}$, respectively. The plot markers are placed in the center of the eccentricity bins.

Figure 4

The upper (lower) panel shows the sensitivity range for the cWB and pycbc search pipelines for equal mass nonspinning, positive spins and negative spins as a function of eccentricity defined at 30 Hz at an $\mathrm{IFAR}>10\mathrm{years}$ ($\mathrm{IFAR}>100\mathrm{years}$). The plot markers are placed in the center of the eccentricity bins.

Figure 5

In the upper and lower panels, the whitened detector response of LIGO Hanford, H1, in time domain is displayed for an injected signal corresponding to the waveform with ID 9 and 12 from Table 1, respectively. Some of the parameters of the injected signals are shown in Table 2. In both panels, the injected and reconstructed signals are represented with blue and orange colors, respectively.

Figure 6

In the upper (lower) panel, the relative difference in sensitivity volume between the search sensitivity of pycbc and cWB for various chirp mass bins is presented as function of eccentricity at an $\mathrm{IFAR}>10$ ($\mathrm{IFAR}>100$) years. The plot markers are placed in the center of the eccentricity bins.