Twin Black Holes: Why Light Ones Are Lazy and Heavy Ones Are Tops express
Two figure skaters glide on the ice. One is light and cautious, barely rotating. The other is massive and fast, spinning with all its might, and in different directions at that. That's roughly the picture painted by gravitational waves from black hole mergers.
Scientists analyzed data from the fourth LIGO–Virgo–KAGRA catalog and found that the spin of black holes in a pair strongly depends on their mass. Light holes, up to 35–50 solar masses, barely spin: their spin (a measure of rotation) does not exceed 0.2 of the maximum possible. Heavy ones, from 70 solar masses and up, spin at full tilt — on average about 0.7 of the limit, and often lopsidedly: one hole in a pair may spin fast while the other is noticeably slower.
The slow spin of light holes fits a scenario where both stars were born together and lived quiet lives, not picking up extra spin. The fast and asymmetric spin of heavy holes looks more like the result of repeated mergers in dense stellar clusters or chaotic captures. However, the expected clear signature of "hierarchical" mergers, where one hole always spins faster than the other, did not show up in the data. Nature, once again, proved more complex.
This spin portrait is a new target for theorists. Now, models of binary black hole formation must explain not only masses but also how those masses relate to the spin of each individual hole.
Original abstract
as written by the authors arXiv:2605.24281
While the origin of merging black-hole binaries observed in gravitational waves remain uncertain, different formation channels are expected to leave distinct imprints on their observed mass and spin distributions. In this work, we focus on the mass dependence of the spin magnitudes $$(\chi_1,\chi_2)$$, allowing for multiple spin subpopulations whose relative fraction varies with mass. Using the binaries from the fourth LIGO--Virgo--KAGRA gravitational wave transient catalog (GWTC-4), we find strong evidence for two subpopulations, with a log Bayes factor of $$\log_{10} \mathcal B = 4^{+6}_{-2}$$. The data support a picture in which low-mass systems are composed almost entirely of slowly spinning black holes ($$\chi \lesssim 0.2$$), while high-mass systems are dominated by a broader component with moderate to rapid spins, peaking at $$(\chi_1,\chi_2)\approx(0.7,0.7)$$ and retaining a non-negligible tail toward asymmetric spins $$(\chi_1,\chi_2)\approx(0.7,0)$$. The transition between these regimes spans a broad range from $$m_1 \approx 35^{+20}_{-20} \,M_\odot$$ to $$m_1 \approx 70^{+90}_{-25}\,M_\odot$$. For the low-mass, slowly-spinning regime, a flexible model of spin orientations does not allow us to exclude identically vanishing spins, $$\chi_1 = \chi_2 = 0$$. Meanwhile, the high-mass, rapidly-spinning population does not neatly display the spin-magnitude asymmetry expected from hierarchical mergers, although the data do not exclude that. This kind of localized structure is generally difficult to access with standard Monte-Carlo likelihood estimators, which we replace with a regularized representation in terms of truncated Gaussian mixtures. Taken together, our results provide a new target for formation models, which should reproduce not only the mass and effective spin distributions, but also the mass-dependent structure of the individual component-spin magnitudes.
A black hole's spin is measured by a number from 0 to 1. A hole with a maximum spin of 1 rotates so fast that its event horizon moves at nearly the speed of light.
In the movie Interstellar, the supermassive black hole Gargantua spins at nearly its maximum spin, which creates the extreme time dilation on Miller's planet.