The Pair Instability Gap

There is a mass range, called the "pair instability gap" in which intermediate mass black holes in binary black hole systems are predicted to be scarce.

Stellar theory predicts a forbidden range of black-hole masses between ∼ 50 - 130 M⊙ due to pair-instability supernovae, but evidence for such a gap in the mass distribution from gravitational-wave astronomy has proved elusive. Early hints of a cutoff in black-hole masses at ∼ 45 M⊙ disappeared with the subsequent discovery of more massive binary black holes. 

Here, we report evidence of the pair-instability gap in LIGO-Virgo-KAGRA's fourth gravitational wave transient catalog (GWTC-4), with a lower boundary of 45 +5 −4 M⊙ (90% credibility). While the gap is not present in the distribution of primary masses m(1) (the bigger of the two black holes in a binary system), it appears unambiguously in the distribution of secondary masses m(2), where m(2) ≤ m(1). The location of the gap lines up well with a previously identified transition in the binary black-hole spin distribution; binaries with primary components in the gap tend to spin more rapidly than those below the gap. 

We interpret these findings as evidence for a subpopulation of hierarchical mergers: binaries where the primary component is the product of a previous black-hole merger and thus populates the gap. Our measurement of the location of the pair-instability gap constrains the S-factor for 12C(α,γ)16O at 300 keV to 256 +197 −104 keV barns.

Hui Tong, et al., "Evidence of the pair instability gap in the distribution of black hole masses" arXiv:2509.04151 (September 4, 2025).