The pair-instability supernova (PISN) mechanism predicts a mass gap in the black hole population, where no stellar-origin black holes are expected to form. However, several binary black hole merger events exhibit component masses that appear to lie within the PISN mass gap. If a gravitational-wave (GW) source is lensed, neglecting lensing effects leads to an underestimation of the luminosity distance and, hence, the redshift, resulting in an overestimation of the source-frame masses and potentially placing them within the mass-gap region. In this work, we analyze two representative events, GW190521 and GW231123. We establish a direct mapping between the lensing magnification and the fraction of posterior samples with source-frame masses below the mass-gap boundary. Adopting a lower bound of $65M_{\odot }$, we find that the magnifications required for 90% of the posterior samples to fall below this boundary are $\mu =444$ for GW231123 and $\mu =39$ for GW190521. At these magnifications, the corresponding source-frame masses of the primary black hole are ${52}_{-10}^{+10}{M}_{\odot }$ and ${42}_{-14}^{+19}{M}_{\odot }$, with lensed source redshifts of ${2.3}_{-0.5}^{+0.8}$ and ${2.7}_{-1.2}^{+1.5}$, respectively. These results provide a quantitative framework for assessing the lensing hypothesis as a possible explanation for binary black hole mergers observed within the PISN mass gap and highlight that the extreme magnifications required for GW231123 may challenge the astrophysical plausibility of simple strong-lensing interpretations.

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• Gravitational lenses
• Gravitational wave sources