Reexamining Evidence of a Pair-Instability Mass Gap in the Binary Black Hole Population

TL;DR

This study uses GWTC-4 data to investigate the black hole mass spectrum, finding no clear evidence of a pair-instability mass gap and suggesting alternative formation channels for high-mass black holes.

Contribution

The paper provides flexible models to analyze the black hole mass and spin distributions, challenging the existence of a sharp mass gap and exploring formation scenarios.

Findings

No evidence of a sharp drop in black hole mass spectrum near 40-50 M_sun.

High-mass, broad-spin sub-population has many systems with mass ratios 0.6-1.

Constraints on the PISN cutoff mass at approximately 57 M_sun.

Abstract

The fourth gravitational wave transient catalog~(GWTC-4) has enabled empirical probes of the theorized pair-instability gap in the higher end of the binary black hole~(BBH) mass-spectrum. In this letter, using flexibly parametrized models, we show that at present there is no evidence of a sharp drop-off in the spectrum of black hole masses near $~40-50M_{\odot}$. We simultaneously characterize the transition in the distribution of BBH mass-ratios, effective aligned and effective precessing spins using our flexible models. From the transitions in our inferred spin and mass-ratio distributions, we find that the high-mass broad-spin sub-population has a significant fraction~($52^{+18}_{-23}\%$) of systems with mass ratios in the range $0.6-1$. This suggests that alternatives to the hypothesis of 2G+1G hierarchical systems dominating BBH formation above $\sim 40-50 M_{\odot}$ are more consistent with the GWTC-4 detection sample. By comparing with the predictions of star cluster simulations, we further show that contributions from (2G+2G) systems are not abundant enough to alleviate this discrepancy. We also demonstrate the effects of strong model assumptions on this inference, which can lead to biased astrophysical interpretation from restricted priors. We note that our results do not exclude that a high-mass gap may be identified as our sample size increases. We constrain the lower bound on the location of a possible PISN cutoff still allowed within measurement uncertainties to be $(57^{+17}_{-10}M_{\odot})$ and discuss its implications on the S factor of $^{12}\mathrm{C}(\alpha, \gamma)^{16}O$ at 300 kev.