[Paper Review] Which black hole formed first? Mass-ratio reversal in massive binary stars from gravitational-wave data

Matthew Mould, Davide Gerosa+2 more|arXiv (Cornell University)|May 24, 2022

Pulsars and Gravitational Waves ResearchPhysics and Astronomy68 references41 citations

TL;DR

This study uses gravitational-wave data from LIGO/Virgo to investigate whether mass-ratio reversal—where the initially less massive star forms the heavier black hole—occurred in massive binary systems. By modeling non-identical spin distributions and subpopulations with negligible spins, the authors find no significant evidence for such systems, constraining the fraction undergoing mass-ratio reversal to be <32% (99% credible interval), while a robust spin peak at χ ≈ 0.2–0.3 remains unexplained by current formation models.

ABSTRACT

Population inference of gravitational-wave catalogues is a useful tool to translate observations of black-hole mergers into constraints on compact-binary formation. Different formation channels predict identifiable signatures in the astrophysical distributions of source parameters, such as masses and spins. One example within the scenario of isolated binary evolution is mass-ratio reversal: even assuming efficient core-envelope coupling in massive stars and tidal spin-up of the stellar companion by the first-born black hole, a compact binary with a lighter, non-spinning first-born black hole and a heavier, spinning second-born black hole can still form through mass transfer from the initially more to less massive progenitor. Using current LIGO/Virgo observations, we measure the fraction of sources in the underlying population with this mass-spin combination and interpret it as a constraint on the occurrence of mass-ratio reversal in massive binary stars. We modify commonly used population models by including negligible-spin subpopulations and, most crucially, non-identical component spin distributions. We do not find evidence for subpopulations of black holes with negligible spins and measure the fraction of massive binary stars undergoing mass-ratio reversal to be consistent with zero and $<32\%$ ($99\%$ confidence). The dimensionless spin peaks around $0.2\unicode{x2013}0.3$ appear robust, however, and are yet to be explained by progenitor formation scenarios.

Motivation & Objective

• To determine whether mass-ratio reversal occurred in massive binary stars, where the initially less massive star forms the heavier black hole.
• To test whether observed gravitational-wave signals from binary black hole mergers exhibit signatures of non-identical spin distributions and negligible-spin subpopulations.
• To constrain the fraction of binary black hole systems that underwent mass-ratio reversal using current LIGO/Virgo observations.
• To assess the robustness of a previously reported overdensity in spin magnitude at χ ≈ 0.2–0.3 under more realistic spin modeling.

Proposed method

• Applied hierarchical Bayesian inference to the LIGO/Virgo GWTC-2.1 catalog of 69 confident binary black hole events.
• Used a modified population model with non-identical spin distributions for primary and secondary black holes, allowing for distinct spin magnitude parameters.
• Incorporated subpopulations with negligible spins via peaks at zero spin, testing their occupation fractions.
• Computed the population likelihood using Monte Carlo integration, reweighting single-event posteriors and detection probabilities from injected signals.
• Sampled the posterior distribution using gwpopulation, bilby, and dynesty, with uninformative priors on population parameters.
• Compared models with and without non-identical spins and negligible-spin components to assess evidence for mass-ratio reversal.

Experimental results

Research questions

• RQ1 What fraction of binary black hole mergers in the LIGO/Virgo catalog show evidence of mass-ratio reversal, as indicated by a heavier, spinning secondary black hole and a lighter, non-spinning primary?
• RQ2 Do current gravitational-wave observations support the existence of a subpopulation of black holes with negligible spins, as predicted by mass-ratio reversal scenarios?
• RQ3 Is the observed overdensity in black hole spin magnitudes around χ ≈ 0.2–0.3 robust when accounting for non-identical spin distributions and negligible-spin subpopulations?
• RQ4 Can the spin configuration of merging black holes be used to infer the formation order of the black holes, particularly whether the heavier one formed second?

Key findings

• The fraction of binary black hole systems undergoing mass-ratio reversal is constrained to be <32% at 99% credible interval, with no significant evidence for such systems in the current data.
• There is no strong support for a subpopulation of black holes with negligible spins; the occupation fraction for primary black holes with zero spin is <46%, and for secondary black holes, <36%.
• The intrinsic fraction of systems where the heavier black hole has significant spin and the lighter one has negligible spin is <32%, interpreted as the MRR fraction.
• The previously reported overdensity in spin magnitude at χ ≈ 0.2–0.3 remains robust even when allowing for non-identical spin distributions and negligible-spin components.
• The posterior distribution shows vanishing support for binaries with both dimensionless spins >0.6, indicating such systems are highly unlikely.
• The results suggest that either mass-ratio reversal is rare in isolated binary evolution or that standard assumptions in isolated binary evolution models may need revision.