LIGO Document P2100239-v9

The population of merging compact binaries inferred using gravitational waves through GWTC-3

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26 Nov 2021, 00:58
We report on the population properties of 76 compact binary mergers detected with gravitational waves below a false alarm rate of 1 per year through the cumulative Gravitational Wave Transient Catalog 3 (GWTC-3). The catalog contains three classes of binary mergers: binary black hole (BBH), binary neutron star (BNS), and neutron star-black hole (NSBH) mergers. We infer the BNS merger rate to be between \( 13\,{\rm Gpc^{-3}\, yr^{-1}} \) and \( 1900\,{\rm Gpc^{-3}\, yr^{-1}} \) and the NSBH merger rate to be between \( 7.4\,{\rm Gpc^{-3}\, yr^{-1}} \) and \( 320\,{\rm Gpc^{-3}\, yr^{-1}} \), assuming a constant rate density versus comoving volume and taking the union of 90% credible intervals for methods used in this work. Accounting for the BBH merger rate to evolve with redshift, we find the BBH merger rate to be between \( 17.3\,{\rm Gpc^{-3}\, yr^{-1}} \) and \( 45\,{\rm Gpc^{-3}\, yr^{-1}} \) at a fiducial redshift (\( z = 0.2 \)). Using both binary neutron star and neutron star-black hole binaries, we obtain a broad, relatively flat neutron star mass distribution extending from \( 1.2^{+0.1}_{-0.2}\, M_\odot \) to \( 2.0^{+0.3}_{-0.2}\, M_\odot \). We can confidently identify a rapid decrease in merger rate versus component mass between neutron star-like masses and black-hole-like masses, but there is no evidence that the merger rate increases again before \( 10 M_\odot \). We also find the binary black hole mass distribution has localized over- and under-densities relative to a power law distribution, with peaks emerging at chirp masses of \( 7.8^{+0.4}_{-0.5}\, M_\odot \) and \( 26.6^{+1.3}_{-1.2}\, M_\odot \). While we continue to find the mass distribution of a binary's more massive component strongly decreases as a function of primary mass, we observe no evidence of a strongly suppressed merger rate above \( \simeq 60 M_\odot \), which would highlight the presence of a upper mass gap. The rate of BBH mergers is observed to increase with redshift at a rate proportional to \( (1 + z)^\kappa \) with \( \kappa = 2.7^{+1.8}_{-1.9} \) for \( z\lesssim 1 \). Observed black hole spins are small, with half of spin magnitudes below \( \chi_i \simeq 0.26 \). We observe evidence of negative aligned spins in the population, and an increase in spin magnitude for systems with more unequal mass ratio. We also observe evidence of misalignment of spins relative to the orbital angular momentum.
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