Considering lensing effect on gravitational wave signals from black holes in mass gap

TL;DR

This paper explores how gravitational lensing can bias the inferred masses of black holes from gravitational wave signals, potentially explaining the presence of black holes in the mass gap predicted by the PISN mechanism.

Contribution

It quantifies the lensing magnifications needed to reconcile observed black hole masses with the PISN mass gap, offering a method to assess lensing as an explanation for mass gap black holes.

Findings

Lensing can significantly increase inferred black hole masses.

Minimum lensing magnifications are 12.2 and 320.1 for GW190521 and GW231123.

Lensing effects could explain black holes appearing in the mass gap.

Abstract

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, the binary black hole merger events GW190521 and GW231123 appear unusual, as current analyses place their component masses within the PISN mass gap. In this work, we investigate the relationship between different lensing magnifications and the inferred source-frame black hole masses for these two events. If the gravitational wave source is lensed, neglecting lensing effect can bias the inferred luminosity distance and hence the redshift, leading to an underestimation of the luminosity distance and consequently an overestimation of the source-frame masses, potentially placing them in the mass-gap region. For the two events in mass gap, when adopting a lower bound of $60 M_{\odot}$ for the mass gap, the minimum magnifications required to shift the inferred source-frame masses below this gap boundary are found to be $\mu=12.2$ for GW190521 and $\mu=320.1$ for GW231123, corresponding to lensing-corrected luminosity distances of $11155\, \mathrm{Mpc}$ and $15207\, \mathrm{Mpc}$, respectively. These results provide a quantitative reference for assessing the lensing hypothesis as a possible explanation for the existence of black holes in the PISN mass gap.