Building Better Spin Models for Merging Binary Black Holes: Evidence for Nonspinning and Rapidly Spinning Nearly Aligned Subpopulations

Abstract

Recent work paints a conflicting portrait of the distribution of black hole spins in merging binaries measured with gravitational waves. Some analyses find that a significant fraction of merging binaries contain at least one black hole with a spin tilt >90° with respect to the orbital angular momentum vector, which has been interpreted as a signature for dynamical assembly. Other analyses find that the data are consistent with a bimodal population in which some binaries contain black holes with negligible spin while the rest contain black holes with spin vectors preferentially aligned with the orbital angular momentum vector. In this work, we scrutinize models for the distribution of black hole spins to pinpoint possible failure modes in which the model yields a faulty conclusion. We reanalyze data from the second LIGO–Virgo gravitational-wave transient catalog (GWTC-2) using a revised spin model, which allows for a subpopulation of black holes with negligible spins. In agreement with recent results by Roulet et al., we show that the GWTC-2 detections are consistent with two distinct subpopulations. We estimate that 69%–90% (90% credible interval) of merging binaries contain black holes with negligible spin χ ≈ 0. The remaining binaries are part of a second subpopulation in which the spin vectors are preferentially (but not exactly) aligned to the orbital angular momentum. The black holes in this second subpopulation are characterized by spins of χ ∼ 0.5. We suggest that the inferred spin distribution is consistent with the hypothesis that all merging binaries form via the field formation scenario.

Rates of compact object coalescences

“…Dynamically formed binary black holes should have isotropically oriented spin directions, while BH-BH mergers from isolated binary evolution are expected to have spins preferentially aligned with the orbital angular momentum (e.g., Farr et al 2017;Rodriguez et al 2016b). Is there already evidence for a separate population of dynamically formed systems based on spin-orbit misalignment angles (Abbott et al 2021c;Roulet et al 2021;Galaudage et al 2021)? Meanwhile, the orbits of most binary black holes are expected to circularise through gravitational-wave emission by the time they approach merger and become detectable with current instruments (Peters 1964).…”

New binary black hole mergers in the LIGO--Virgo O3a data

“…Under the current mass prior, however, which is the same uniform-indetector-masses prior used in other catalogs [3,5], the BBH solution with a secondary in the LMG and a mass ratio of ∼ 0.5 is favored over the NSBH solution with mass ratio ∼ 0.2. In either case the effective spin is positive, which (along with the masses) makes these systems feasible to produce in standard isolated binary evolution models [72,73,92]. Since the maximum NS mass is uncertain (according to Alsing et al [61] it can be constrained to [2, 2.6] M ), determining whether these are BBH or NSBH mergers may be sensitive to prior assumptions.…”

“…The mass distribution is coupled to the spin distribution in many formation channel models (see, e.g., discussion by Mapelli [88]) -especially near mass gap edges [89] -and correlation between masses and spins has been found in the detected population [90]. Indeed, population models which allow for this mass-spin correlation are significantly better at fitting the population than models which do not [91], as are models which allow for independently modeled sub-populations [19,92]. Included in the new events reported here are examples of well-measured large effective spins in both directions (see Table I), which will improve statistics in the ongoing empirical analyses of the population's underlying spin distribution.…”

When models fail: an introduction to posterior predictive checks and model misspecification in gravitational-wave astronomy

“…It is worth noting that a model can pass this test while still being quite badly misspecified. Abbott et al (2021d) found evidence for negatively aligned spins in a population of compact-object binaries, but this was later shown to be a model-dependent feature (Roulet et al, 2021;Galaudage et al, 2021). This test may be particularly unreliable if there is a sharp feature in the data that is not captured by the prior.…”