Binary Evolution Can Mimic the Pair-Instability Mass Gap in Black Hole Mergers

The recent O4 catalogs from the LIGO-Virgo-KAGRA collaboration, which significantly increased the number of gravitational-wave (GW) detections, reveal features with potentially important astrophysical implications. One notable example is a hint of the so-called pair-instability mass gap. In particular, the observed decline in the number of black holes (BHs) with mass above 45Msun, together with indications of possibly higher spins for BHs above this threshold, has been interpreted by Antonini et al. and Tong et al. as evidence for pair-instability supernovae.In this work, we investigate whether mass transfer in binary systems can produce features in the BH component-mass distributions that mimic the inferred pair-instability limit. We use both the population synthesis code StarTrack and a simple semi-analytical framework to highlight the impact of mass transfer efficiency on the BH masses. We find that mass-ratio reversal during the first Roche-lobe overflow, followed by a highly non-conservative second mass-transfer phase, naturally limits the mass of the first-born BH and produces an apparent cutoff that mimics the lower edge of a mass gap. While the upper mass limit for the more massive BH is increased through accretion during the first mass-transfer phase and is ultimately set by the pair-instability limit, the less massive BH is limited to the stripped primary mass. As a result, the fraction of systems in which the less massive BH exceeds 45Msun is negligible, below 0.0001 in our default models. While pair instability remains a possible interpretation of current GW data, similar features can naturally arise from binary evolution without invoking a low pair-instability cutoff. The emerging evidence for predominantly positive effective spins extending into the putative upper mass gap may further support a contribution from isolated binary evolution to the high-mass tail.