[Paper Review] Constraints on compact binary merger evolution from spin-orbit misalignment in gravitational-wave observations

B. P. Gompertz, M. Nicholl+3 more|arXiv (Cornell University)|Aug 23, 2021

Pulsars and Gravitational Waves ResearchPhysics and Astronomy125 references28 citations

TL;DR

This paper investigates spin-orbit misalignment in neutron star-black hole (NS-BH) gravitational wave events to constrain binary evolution mechanisms. Using posterior distributions of tilt angles from LIGO-Virgo-KAGRA detections, it finds significant anti-alignment in GW200115, GW190426_152155, and GW190917_114630, challenging standard isolated binary evolution models and suggesting strong natal kicks or stable mass transfer. These findings imply efficient mass transfer and isotropic kicks are key in NS-BH formation.

ABSTRACT

The identification of the first confirmed neutron star - black hole (NS-BH) binary mergers by the LIGO, Virgo and KAGRA collaboration provides the opportunity to investigate the properties of the early sample of confirmed and candidate events. Here, we focus primarily on the tilt angle of the black hole's spin relative to the orbital angular momentum vector of the binary, and the implications for the physical processes that determine this tilt. The posterior tilt distributions of GW200115 and the candidate events GW190426_152155 and GW190917_114630 peak at significantly anti-aligned orientations (though display wide distributions). Producing these tilts through isolated binary evolution would require stronger natal kicks than are typically considered (and preferentially-polar kicks would be ruled out), and/or an additional source of tilt such as stable mass transfer. The early sample of NS-BH events are less massive than expected for classical formation channels, and may provide evidence for efficient mass transfer that results in the merger of more massive NS-BH binaries before their evolution to the compact phase is complete. We predict that future gravitational-wave detections of NS-BH events will continue to display total binary masses of $\approx 7$ M$_{\odot}$ and mass ratios of $q \sim 3$ if this interpretation is correct. Conversely, the high mass of the candidate GW191219_163120 suggests a dynamical capture origin. Large tilts in a significant fraction of merging NS-BH systems would weaken the prospects for electromagnetic detection. However, EM observations, including non-detections, can significantly tighten the constraints on spin and mass ratio.

Motivation & Objective

• To understand the physical mechanisms behind spin-orbit misalignment in early NS-BH gravitational wave events.
• To test whether isolated binary evolution or dynamical formation best explains observed tilt angle distributions.
• To assess the implications of spin misalignment for electromagnetic follow-up and multi-messenger astronomy.
• To constrain mass transfer efficiency and natal kick properties in NS-BH binary evolution.

Proposed method

• Constructed posterior predictive distributions (PPD) for black hole spin tilt angles using public GW posterior samples from GWTC-2, GWTC-2.1, and GWTC-3.
• Compared NS-BH tilt angle posteriors to the full population of binary black hole (BBH) mergers from Abbott et al. (2021d) to assess statistical consistency.
• Evaluated the impact of isotropic vs. polar natal kicks on spin-orbit misalignment, using simulations and theoretical models.
• Assessed the role of stable mass transfer in inducing spin misalignment via torque on the donor star.
• Used Bayesian inference to quantify tension between observed tilt distributions and canonical natal kick models.
• Predicted future NS-BH detection trends based on inferred evolutionary pathways.

Experimental results

Research questions

• RQ1 Are the observed spin-orbit misalignments in NS-BH mergers consistent with isolated binary evolution models?
• RQ2 What physical mechanisms—such as natal kicks or mass transfer—can explain the observed anti-aligned spin distributions?
• RQ3 How do the observed tilt angles constrain the isotropy and magnitude of black hole natal kicks?
• RQ4 To what extent do the low total masses and high mass ratios of early NS-BH events indicate efficient mass transfer before core collapse?
• RQ5 What are the implications of large spin-orbit misalignments for electromagnetic counterparts of NS-BH mergers?

Key findings

• The posterior distributions for GW200115, GW190426_152155, and GW190917_114630 peak at significantly anti-aligned spin-orbit orientations, indicating strong misalignment.
• These anti-aligned tilts are inconsistent with standard isolated binary evolution unless natal kicks exceed typical values or additional tilting mechanisms like stable mass transfer are invoked.
• The observed low total binary masses (~7 M⊙) and high mass ratios (q ≈ 3) suggest efficient mass transfer prior to the first supernova, favoring non-classical formation channels.
• Isotropic natal kicks are favored over polar kicks, as the latter fail to produce the observed tilt distributions.
• The high spin misalignment in a significant fraction of NS-BH systems reduces prospects for electromagnetic counterparts, especially if the BH has anti-aligned spin.
• Future GW-EM joint observations can tighten constraints on spin and mass ratio, breaking degeneracies in current measurements.