GW231123: False Massive Graviton Signatures from Unmodeled Point-Mass Lensing

TL;DR

This paper demonstrates that unmodeled point-mass lensing can mimic signals of nonzero graviton mass in gravitational wave data, affecting tests of gravity and confirming GW231123 as a lensed event.

Contribution

It shows how unmodeled lensing effects can produce false signals of modified gravity, emphasizing the need to account for lensing in gravitational wave analyses.

Findings

Unmodeled lensing explains the apparent nonzero graviton mass in GW231123.

Lensed injections with zero graviton mass produce similar anomalies in analysis.

Including lensing in models removes the false graviton mass signal.

Abstract

GW231123 is the strongest current candidate for a lensed gravitational wave event and a unique case for testing how point-mass lensing affects propagation-based tests of gravity. In the real GW231123 data, an unlensed IMRPhenomXPHM analysis yields an apparent nonzero graviton mass posterior. We show that this anomaly is naturally explained by unmodeled point-mass lensing: once lensing is included, the apparent graviton mass signal disappears. In GW231123-like injection-recovery tests, a lensed NRSur7dq4 signal with zero graviton mass, recovered with the same unlensed IMRPhenomXPHM template, produces a similarly pronounced spurious graviton mass posterior, whereas lensing-included analyses with IMRPhenomXPHM, IMRPhenomXO4a, and NRSur7dq4 remain mutually consistent with no evidence for nonzero graviton mass. The similarity between the injected and real data posteriors shows that unmodeled point-mass lensing can mimic modified gravitational wave propagation. These results identify a concrete failure mode in tests of gravity and strengthen the interpretation of GW231123 as a lensed candidate.