What is the most massive gravitational-wave source?

Abstract

In the presence of significant measurement uncertainties, the events which appear to be the most extreme are very likely to be those exhibiting the greatest statistical fluctuations. It is therefore particularly important to exercise care when interpreting such events and to use the entire observed population for context. Here, I attempt to pedagogically illustrate this using the example of the most massive binary black hole so far detected in gravitational-wave data, GW231123. I argue that its total mass may be significantly lower than $238^{+28}_{-49}$ solar masses as reported by Abac et al. (2025a). The maximum total binary black hole mass from an analysis of the entire detected population is below 170 solar masses if the same priors that are used for individual event analyses in the GWTC catalogs, including for the analysis of GW231123, are applied to the population as a whole. However, this value is very sensitive to assumptions about the population distribution.