Impact of the wave-like nature of Proca stars on their gravitational-wave emission

TL;DR

This study systematically investigates how the wave-like properties of Proca stars influence their gravitational-wave emission during head-on collisions, revealing unique phase-dependent effects not seen in black-hole mergers.

Contribution

It provides a comprehensive numerical-relativity catalogue of Proca star collisions and uncovers the significant impact of the initial phase on gravitational-wave signals, highlighting novel phenomenology.

Findings

Wave-like nature significantly affects gravitational-wave emission.

Initial phase strongly influences emitted energy and mode structure.

Emission depends non-monotonically on secondary star frequency.

Abstract

We present a systematic study of the dynamics and gravitational-wave emission of head-on collisions of spinning vector boson stars, known as Proca stars. To this aim we build a catalogue of about 800 numerical-relativity simulations of such systems. We find that the wave-like nature of bosonic stars has a large impact on the gravitational-wave emission. In particular, we show that the initial relative phase $\Delta \epsilon =\epsilon_1-\epsilon_2$ of the two complex fields forming the stars (or equivalently, the relative phase at merger) strongly impacts both the emitted gravitational-wave energy and the corresponding mode structure. This leads to a non-monotonic dependence of the emission on the frequency of the secondary star $\omega_2$, for fixed frequency $\omega_1$ of the primary. This phenomenology, which has not been found for the case of black-hole mergers, reflects the distinct ability of the Proca field to interact with itself in both constructive and destructive manners. We postulate this may serve as a smoking gun to shed light on the possible existence of these objects.