GWTC-4.0: Population Properties of Merging Compact Binaries

TL;DR

GWTC-4.0 expands the population picture of merging compact binaries with 158 events, applying hierarchical Bayesian inference to jointly constrain masses, spins, and redshift evolution under both strongly modeled (parametric) and weakly modeled (nonparametric) population frameworks. The analysis confirms persistent mass-spectrum features around $1$–$2\,M_\odot$, $\sim10\,M_\odot$, and $\sim35\,M_\odot$, with hints of a third feature near $\sim20\,M_\odot$ in flexible models, and finds a non-negligible fraction of binaries with negative $\chi_{\rm eff}$, implying both isolated and dynamical formation channels. BBH spin distributions show non-extremal magnitudes with a tilt structure indicating partial alignment, while the $\chi_{\rm eff}$ distribution is skewed toward positive values, and a mild $q$–$\chi_{\rm eff}$ correlation is favored in several models. The inferred BBH redshift evolution is broadly consistent with cosmic star-formation-rate-driven expectations, though there is evidence for broader spin distributions at higher redshift and signs of multiple subpopulations. Collectively, these results refine the mass-gap landscape, illuminate formation channels, and set the stage for upcoming data from the remainder of O4 to further illuminate compact-binary evolution and demographics.

Abstract

We detail the population properties of merging compact objects using 158 mergers from the cumulative Gravitational-Wave Transient Catalog 4.0, which includes three types of binary mergers: binary neutron star, neutron star--black hole binary, and binary black hole mergers. We resolve multiple over- and under-densities in the black hole mass distribution: features persist at primary masses of $10\,M_\odot$ and $35\,M_\odot$ with a possible third feature at $\sim 20\,M_\odot$. These are departures from an otherwise power-law-like continuum that steepens above $35\,M_\odot$. Binary black holes with primary masses near $10\,M_\odot$ are more likely to have less massive secondaries, with a mass ratio distribution peaking at $q = 0.74^{+0.13}_{-0.13}$, potentially a signature of stable mass transfer during binary evolution. Black hole spins are inferred to be non-extremal, with 90\% of black holes having $χ< 0.57$, and preferentially aligned with binary orbits, implying many merging binaries form in isolation. However, we find a significant fraction, 0.24-0.42, of binaries have negative effective inspiral spins, suggesting many could be formed dynamically in gas-free environments. We find evidence for correlation between effective inspiral spin and mass ratio, though it is unclear if this is driven by variation in the mode of the distribution or the width. (Abridged)

Figures (22)

• Figure 1: The complete mass spectrum, inferred using the strongly modeled FullPop-4.0 and weakly modeled BGP analyses. In the lower triangle of the $m_1$--$m_2$ plane (where $m_1 > m_2$ by definition), we show median merger rate density across primary and secondary masses. At $m_2 = 5\,\mathit{M_\odot}$, the FullPop-4.0 model transitions to an alternative pairing function to allow for NSBH, hence the discontinuity at $m_2 = 5\,\mathit{M_\odot}$. In the upper triangle we show the fractional uncertainty in the merger rate reflected across the diagonal. The fractional uncertainty $\Delta\mathcal{R} /\mathcal{R}$ is difference of the 95th and 5th percentile values divided by the median merger rate. Both the FullPop-4.0 and BGP models show similar broad features: a population of BNS at primary and secondary masses $\lesssim 2\,\mathit{M_\odot}$, a population of NSBH at primary mass $\sim 9\,\mathit{M_\odot}$ and secondary mass $\lesssim 2\,\mathit{M_\odot}$, and finally the population of BBH with primary and secondary masses $\gtrsim 9\,\mathit{M_\odot}$. The uncertainties are the smallest in the $\sim 9\,\mathit{M_\odot}$ and $\sim 30\,\mathit{M_\odot}$BBH peaks.
• Figure 2: A comparison of the merger rate at redshift $z=0$, as a function of component mass for the FullPop-4.0 and BGP models. In the upper panel, we show the merger rate as a function of primary mass, marginalized over secondary mass. In the lower panel, we show the merger rate as a function of the secondary mass, marginalized over the primary mass. At $m_2 = 5\,\mathit{M_\odot}$, the FullPop-4.0 model transitions to an alternative pairing function to allow for NSBH, hence the discontinuity in the secondary mass. The median of the inferred merger rate is shown with a solid line, and the 90% credible interval is shown in the shaded region. The BGP weakly modeled approach has larger uncertainties due to increased flexibility. Both the strongly modeled and weakly modeled approaches find local maxima in the merger rate at $m_2 \sim 1-2 \,\mathit{M_\odot}$ and $m_2 \sim 6.5-9\,\mathit{M_\odot}$ in the secondary-mass distribution. However, the BGP model does not confidently recover the low mass peaks in the primary-mass distribution. In the inset, we show the FullPop-4.0 inferred gap depth parameter $A$ (see Appendix \ref{['appendix:PDB']}), where $A=1$ (a completely empty gap) is disfavored.
• Figure 3: Differential merger rate as a function of primary mass (evaluated at $z=0.2$) of the Broken Power Law + 2 Peaks model (orange) and B-Spline model (blue) compared to the Power Law + Peak model from GWTC-3.0 KAGRA:2021duu. The solid lines indicate the posterior medians and the shaded regions show the 90% credible interval of each model. Comparing these results, it is clear that a single power law is a poor description of the the low-mass end of the spectrum.
• Figure 4: Differential merger rate as a function of primary mass (evaluated at $z=0.2$) of the Broken Power Law + 2 Peaks model inferred with GWTC-4.0 compared to the same model applied to only GWTC-3.0 BBH events. The orange shaded region shows the 90% credible interval for GWTC-4.0 and the solid orange curve shows the posterior median, while the black dashed curves bound the $90\%$ credible region for GWTC-3.0 and the solid black curve shows the posterior median. The inferred distribution is similar between catalogs, which highlights that the low-mass structure identified in GWTC-4.0 was present in GWTC-3.0. The inset figure shows the joint posterior of the broken power law index parameters $\alpha_1$ and $\alpha_2$ for GWTC-3.0 (gray) and GWTC-4.0 (orange), with the contours showing the $5^{\rm th}, 50^{\rm th}$, and $95^{\rm th}$ percentiles. The black dashed curve in the inset indicates where $\alpha_1 = \alpha_2$.
• Figure 5: Differential merger rate as a function of mass-ratio (evaluated at $z=0.2$) of the Broken Power Law + 2 Peaks model (orange) and B-Spline model (blue). The fiducial Power Law + Peak model from KAGRA:2021duu is included for comparison. Solid curves indicate posterior medians and the shaded (dashed) regions show 90% credible intervals.