Identifying Proca-star mergers via consistent ultralight-boson mass estimates across gravitational-wave events

Abstract: While black-hole and neutron-star mergers are the most plausible sources of current gravitational-wave observations, mergers of exotic compact objects may mimic these signals. Proca stars -- Bose-Einstein condensates of complex vector ultralight bosons -- have gained significant attention for their potential to replicate certain gravitational-wave events while yielding consistent estimates of the boson mass $\mu_{B}$ forming the stars. Using a mixture model within a Bayesian framework, we demonstrate that consistent boson-mass estimates across events can yield conclusive evidence for the existence for a number $n$ of Proca-star families characterized by respective boson masses $\mu_{B}^{i}$, even if no individual event can be conclusively identified as such. Our method provides posterior distributions for $n$ and $\mu_{B}^{i}$. Applying this framework to the high-mass events GW190521, GW190426_190642, GW200220_061928, we obtain a Bayes Factor ${\cal{B}}^{n=0}_{n=1} \simeq 2$ against the Proca-star hypothesis, primarily rooted in the limitation of current Proca-star merger simulations to intrinsically weak head-on cases. We discuss the use of priors that mitigate the impact of these limitations, which yields ${\cal{B}}^{n=1}_{n=0} \simeq 8$ in favour of a single Proca family hypothesis; and the inclusion of the gravitational-wave trigger S200114$_$020818. Finally, we show that conclusive evidence $\log{\cal{B}}^{n=1}_{n=0} \geq 5$ could be achieved after 5 to 9 observations of similar event sets, at the $90\%$ credible level. This method provides a new way to detect exotic compact objects, somewhat using gravitational-wave detectors as particle detectors.