What is the most massive gravitational-wave source?

TL;DR

The paper investigates how to robustly infer the maximum total mass of detectable binary black holes given LVK priors and measurement uncertainties, with a focus on GW231123. It shows that applying LVK priors under a hierarchical model yields a population-maximum mass of $M_{max}\approx 167^{+4}_{-1}$ $M_ obreakdash_ ext{odot}$, which is well below the event's reported $238^{+28}_{-49}$ $M_ obreakdash_ ext{odot}$, suggesting GW231123 may be an extreme statistical fluctuation rather than a genuinely exceptional population. The analysis demonstrates that the inferred $M_{max}$ is highly sensitive to the assumed population tail, as alternative priors can push $M_{max}$ up to the $190$–$249$ $M_ obreakdash_ ext{odot}$ range, underscoring the importance of population modeling and selection effects. Overall, the work cautions against over-interpreting extreme single events without a full population-context, and highlights the need for additional observations and refined hierarchical models to determine whether such high-mass events reflect real tails or artefacts of limited data.

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.

Figures (2)

• Figure 1: The posterior probability distribution on the maximum total mass $M_{max}$ given the LVK data from GWTC1 through GWTC4 (blue) and the histogram of the LVK "mixed" posterior on the source-frame total mass of GW231123 from a combination of analyses with different gravitational waveforms (orange, scaled up by a factor of 10 for improved visibility).
• Figure 2: The cumulative distribution of total masses (thin colored background lines), the cumulative distribution of median total masses (blue line), the analytical cumulative distribution function from the flat-component-mass prior using $M_{max}=167$ M$_\odot$ (dashed black line) and cumulative distribution functions of draws from this model containing the number of observed events in the GWTC catalogs (grey lines).