Insights from Misaligned Black Hole Pairs

Astronomers study gravitationally linked pairs of black holes to understand how stellar-mass black holes form. What can we learn from black holes that are off kilter from the binaries they belong to?

You Spin Your Way, and I’ll Spin Mine

Collecting the gravitational waves from merging stellar-mass black holes allows us get at one of the fundamental questions in high-energy astrophysics: how did the black holes that exist in the universe today come to be? A black hole’s mass and spin hint at whether it formed directly from the collapse of a massive star or through a process called hierarchical merging — the formation of large black holes through successive mergers of smaller ones.

One property we can use to probe the origins of black holes in merging binary systems is the effective inspiral spin parameter, denoted χ_eff, which is a measure of how aligned the spins of the black holes are with the orbit of the binary pair. In some systems, the black holes spin in the same direction as they orbit, giving the system positive χ_eff. In others, one or both black holes are tilted, and they spin in directions that are at odds with their orbital motion, giving the system negative χ_eff. A recent publication searches for the most misaligned black hole binaries to understand the origins of the black holes in the universe today.

Monitoring Misalignment

Maya Fishbach (Northwestern University) and collaborators began their investigation by predicting the spins of black holes in binaries in which at least one member formed through hierarchical merging. The team’s calculations showed that hierarchical merging results in a substantial number of black holes with spins misaligned from their direction of orbital motion. Specifically, the team predicted that 16% of black hole binaries would have χ_eff less than −0.3, regardless of whether one or both members of the binary system arose through hierarchical merging.

With this prediction in hand, Fishbach and coauthors turned to the data, assessing the χ_eff values of 69 merging black hole pairs in the third Gravitational-wave Transient Catalog (GWTC-3), which is the most recent catalog of gravitational wave events detected by LIGO and Virgo. Depending on the model used to analyze the data, the team found that the maximum proportion of binary systems with χ_eff less than −0.3 is 4.2%.

Evidence for Meager Mergers

Given the small fraction of strongly misaligned black hole systems, the team found that hierarchical mergers likely populate no more than 26% of black hole binaries. Intriguingly, only 69% of massive black holes (60 solar masses) are expected to be products of hierarchical merging, meaning that some of these high-mass black holes most likely result from enormous collapsing stars — a finding that has implications for our understanding of stellar interiors and nuclear physics.

Fishbach and collaborators note that if black hole binaries with χ_eff less than −0.3 aren’t detected in the future, that would point to hierarchical merging being even less common. If the LIGO, Virgo, and KAGRA gravitational wave detectors find no such systems during their next observing runs, the team limits the percentage of black holes formed through hierarchical merging to no more than 2.5%.

Citation

“Limits on Hierarchical Black Hole Mergers from the Most Negative χ_eff Systems,” Maya Fishbach et al 2022 ApJL 935 L26. doi:10.3847/2041-8213/ac86c4