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

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 parameterized models, we show that, at present, there is no evidence of a sharp drop-off in the spectrum of black hole masses near 40–50 M ⊙ . We simultaneously characterize the transition in the distribution of BBH mass ratios and 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 subpopulation has a significant fraction ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>5</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>23</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>18</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>%</mml:mo> </mml:math> ) 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 ∼40–50 M ⊙ 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">(</mml:mo> <mml:mn>5</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>17</mml:mn> </mml:mrow> </mml:msubsup> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:math> and discuss its implications on the S factor of 12 C( α , γ ) 16 O at 300 kev.