Nonparametric perspective on correlations in the gravitational-wave population through GWTC-4 with PixelPop

As gravitational-wave observatories continue to collect data, statistically significant structures and correlations are emerging within the population of compact binaries. With the release of the LIGO–Virgo–KAGRA Collaboration's fourth gravitational-wave catalog, the number of observed compact binary mergers has grown substantially. We apply PixelPop, a weakly modeled, nonparametric framework for high-resolution reconstruction of population correlations, to update our understanding of the underlying astrophysical distributions. From a data-driven perspective, we examine (1) the population near the neutron star–black hole transition, (2) possible correlations between black hole masses and spins, and (3) features and gaps in the mass spectrum. We conclude with an astrophysical interpretation of these results and discuss recent technical developments in the PixelPop model.