New gravitational-wave data support a bimodal black-hole mass distribution

TL;DR

This paper presents evidence from gravitational-wave data supporting a bimodal distribution of black-hole masses, which aligns with stellar evolution models and explains observed merger mass patterns.

Contribution

It demonstrates that a bimodal black-hole mass model best reproduces the observed distribution of merging black hole masses in gravitational-wave data.

Findings

Bimodal mass distribution explains observed chirp mass peaks.

Only the bimodal model matches the structure of observed data.

Other mass models fail to reproduce the observed distribution.

Abstract

Detailed stellar evolution and supernova models yield a bimodal black-hole mass distribution with a narrow peak around 10 solar masses from stars within a narrow range of progenitor properties and a second broader peak starting around 20 solar masses from very massive progenitors. This bimodal black-hole mass distribution leads to a characteristic distribution of chirp masses of merging binary black holes, with two main peaks arising from the merger of two black holes where both come either from the low- or the high-mass peak and a smaller peak in between from the mixed merger of a low-mass and a high-mass black hole. We carry out a population synthesis study of binary black hole formation and compare the results to the observed chirp masses of gravitational-wave events. We find that only the bimodal black-hole mass prescription is able to reproduce the structure of peaks and gaps in the observed chirp-mass distribution, which is not matched by predictions from other remnant mass prescriptions in the literature.

Full text

New gravitational-wave data support

a bimodal black-hole mass distribution

Features in the GWTC-4 chirp-mass distribution

cannot be explained by traditional remnant-mass prescriptions

R. Willcox1,2 reinhold.willcox@kuleuven.be

F. R. N. Schneider3,4,5

E. Laplace1,2,3,6

Ph. Podsiadlowski7,8,3

K. Maltsev3

I. Mandel9,10

P. Marchant11

H. Sana1,2

T. Li1,2

T. Hertog2,12

(August 28, 2025)

**There has been dramatic recent progress in our understanding of the deaths of massive stars (Janka et al., 2007; Müller, 2020; Heger et al., 2023; Burrows et al., 2024). Detailed stellar evolution studies find a robust, bimodal pattern in the final structures of massive stars (Sukhbold & Woosley, 2014; Patton et al., 2022; Chieffi & Limongi, 2020; Takahashi et al., 2023; Temaj et al., 2024) whose origin has recently been traced back to their late neutrino-dominated nuclear burning (Laplace et al., 2025). Based on predictions from a simplified neutrino-driven supernova explosion model (Müller et al., 2016), these patterns can be linked to the formation of black holes. Applying these to isolated binary stars, Schneider et al. (2023) predict a bimodal black-hole mass distribution with a narrow peak around $10~M_{\odot}$ from stars within a narrow range of progenitor properties and a second broader peak starting around $20~M_{\odot}$ from very massive progenitors. This bimodal black-hole mass distribution leads to a characteristic distribution of chirp masses $\mathcal{M}=M_{1}^{3/5}M_{2}^{3/5}(M_{1}+M_{2})^{-1/5}$ of merging binary black holes, with two main peaks arising from the merger of two black holes where both come either from the low- or the high-mass peak and a smaller peak in between from the mixed merger of a low-mass and a high-mass black hole (Schneider et al., 2023).

**

The recent Gravitational-Wave Transient Catalog Data Release 4 (GWTC-4, Abac et al. 2025) doubles the number of observed compact object mergers with false-alarm rate $<1~\mathrm{yr}^{-1}$. This brings to 158 the total sample of confident events with reported chirp-mass measurements ranging between $\sim$1.2 and $\sim$100 $M_{\odot}$. We focus here on the range $5<\mathcal{M}/M_{\odot}<30$, which showed a tentative gap in the chirp-mass distribution around $10~M_{\odot}$ in the previous data release, GWTC-3 (Abbott et al., 2023). Combined with the new data, the observed source-frame chirp-mass distribution now shows a clear peak around $8~M_{\odot}$, a prominent gap at $10~M_{\odot}$ and a rise again up to $\sim$27 $M_{\odot}$; there may also be a smaller peak around $13~M_{\odot}$ and a dearth between $\sim$15–$20~M_{\odot}$ (see Figure 1). We focus our comparison on the chirp mass because in this range it is better measured than individual masses, whose uncertainty prevents robust feature identification (Adamcewicz et al., 2024; Galaudage & Lamberts, 2025).

The observed chirp-mass distribution strongly resembles population synthesis predictions utilizing the Bimodal BH model (see Figure 1). These population synthesis predictions are based on a new remnant-mass model (Maltsev et al., 2025), which evaluates several pre-explosion variables obtained from detailed stellar evolution models (Schneider et al., 2021, 2023, 2024; Temaj et al., 2024) to anticipate the final fate of massive stars. The predictions follow the Müller et al. (2016) semi-analytical model and yield a non-monotonic function of carbon-oxygen core mass, metallicity, and mass loss history, including mass transfer onto a binary companion.

In an upcoming publication (Willcox et al. in prep), we describe the implementation of this bimodal black-hole formation model into the rapid population synthesis code COMPAS (Team COMPAS: Riley et al., 2022; Team COMPAS: Mandel et al., 2025) to predict the intrinsic and detectable chirp-mass distributions of merging binary black holes (see Figure 2).

On the other hand, the predictions from other remnant-mass prescriptions in the literature do not reproduce the peak-gap structure of the gravitational-wave data (see Figure 1).

The gravitational-wave data show a well defined peak around $8M_{\odot}$, which is slightly lower than the predicted peak, possibly suggesting slightly more mass loss in the progenitor evolution. The high-mass predicted peak in the bimodal black-hole mass model is more significant than in the data. This peak is caused by systems that underwent stable mass transfer after the formation of the first black hole. The rate is very sensitive to the details of the treatment of stable mass transfer, in particular, how mass is lost from the system.

If confirmed, a bimodal black-hole mass distribution provides a key observational constraint on the core-collapse supernova mechanism. It may also be used to constrain the evolution of massive stars, as well as uncertain nuclear physics and fundamental binary physics. Meanwhile, robust, redshift-dependent features of the chirp-mass distribution can be utilized as standard sirens to constrain models of cosmological expansion (Schneider et al., 2023).