Two dead stars circle in a final dance. Their approach creates ripples in space — gravitational waves. But there may be a third, invisible partner in this dance. Its gravity accelerates the pair, and the signal changes pitch slightly, like the siren of a passing car.

The authors of the study learned to catch this pitch shift — the acceleration of the center of mass (LOSA). They incorporated the correction into two mathematical models of mergers: one for circular orbits with spin, the other for elongated, eccentric orbits. Tests on synthetic signals showed that the eccentric model honestly finds acceleration, while the circular one can be fooled and mistake orbital elongation for acceleration.

Then the scientists took five real merger events of neutron stars with black holes from the LIGO-Virgo-KAGRA catalog. For three of them, constraints on acceleration were set for the first time. All five are generally consistent with zero acceleration. But for event GW200105_162426, the joint analysis of acceleration and eccentricity showed that the data are in poor agreement with both parameters being zero simultaneously. This strengthens hints of an elongated orbit noticed earlier.

Thus, gravitational waves become not just messengers of catastrophes, but detectives capable of reconstructing the crime scene: whether the pair was alone or pushed by an invisible neighbor.