Exceptionality of exceptional gravitational-wave events

Abstract

In gravitational-wave astronomy, as in other scientific disciplines, ``exceptional'' sources attract considerable interest because they challenge our current understanding of the underlying (astro)physical processes. Crucially, ``exceptionality'' is defined only relative to the rest of the detected population. For instance, among all gravitational-wave events detected so far, GW231123 is the binary black hole with the largest total mass, while GW241110 is the binary black hole with the most strongly misaligned spin relative to the orbital angular momentum. Mandel [Astrophys. J. Lett. 996, L4 (2026)] argued that apparent ``exceptionality'' may reflect measurement error rather than an extreme true value, and suggested that the total mass of GW231123 may be significantly overestimated. Here we present a quantitative analysis that supports this conceptual point. We find that claims of ``exceptionality'' obtained under population-agnostic priors should be critically questioned whenever measurement uncertainties are comparable to the width of the underlying population. Specifically, we find that the total mass of GW231123 is unlikely to be meaningfully affected by this effect while the spin of GW241110 is far less likely to be antialigned than initially claimed: about 70% of realizations that appear to yield an ``exceptionally antialigned'' spin are in fact consistent with either nonspinning or aligned configurations.

4 pages, 4 figures

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