Astrophysics informed Gaussian processes for gravitational-wave populations: Evidence for the onset of the pair-instability supernova mass gap

TL;DR

This paper uses Gaussian Processes to analyze gravitational-wave data, providing evidence for the pair-instability supernova mass gap in black hole populations and identifying a potential hierarchical merger subpopulation.

Contribution

It introduces a flexible, non-parametric GP framework to detect complex features like the mass gap in black hole mass distributions from gravitational-wave data.

Findings

Strong evidence for the onset of the PISN mass gap at 45-60 M_sun

Suppression of merger rate by a factor of 10-60 near 60 M_sun

Identification of a subpopulation around 70 M_sun likely due to hierarchical mergers

Abstract

We analyze binary black hole (BBH) mergers from the latest Gravitational Wave Transient Catalog (GWTC-3) using a flexible, non-parametric framework to infer the underlying black hole mass distribution. Our model employs Gaussian Processes (GPs) with astrophysically motivated priors to represent the mass distribution, assuming both component masses are independently drawn from a common mass function. This approach enables us to capture complex features in the population without relying on rigid parametric forms. Motivated by predictions from binary stellar evolution, we focus on the presence of a mass gap in the range $40-120~ M_\odot$, attributed to the pair-instability supernova (PISN) and pulsational PISN (PPISN) processes. Using our GP-based model, we find for the first time, strong evidence for the onset of this mass gap, locating its lower edge at approximately $45-60 ~M_\odot$. We observe a suppression in the BBH merger rate when comparing the latest constraints against our GP model, by a factor of $10-60$ at $\sim60~M_\odot$, corresponding to the minimum of the mass gap. Additionally, we find evidence of a subpopulation of mergers populating the mass gap around $70 ~M_\odot$, which we argue is due to hierarchical mergers, as well as of a feature in the $40-50~ M_\odot$ range, albeit with low significance, which we attribute to the predicted PPISN build up. We discuss the astrophysical implications of this result in light of GW231123, a recently reported BBH merger with a total mass potentially above the PISN gap, suggesting the need for revised models of massive stellar evolution or alternative formation channels.