Binary boson stars: Merger dynamics and formation of rotating remnant stars

TL;DR

This paper investigates the dynamics of binary boson star mergers, highlighting how scalar interactions influence gravitational wave signals and demonstrating the formation of rotating boson star remnants, with implications for understanding ultra compact objects.

Contribution

It introduces a new numerical framework for binary boson star mergers, revealing the impact of scalar interactions on merger dynamics and remnant properties, including the formation of rotating boson stars from non-spinning pairs.

Findings

Scalar interactions significantly alter gravitational wave signals.

Rotating boson stars can form from non-spinning binary mergers.

A mapping predicts remnant properties across different merger scenarios.

Abstract

Scalar boson stars have attracted attention as simple models for exploring the nonlinear dynamics of a large class of ultra compact and black hole mimicking objects. Here, we study the impact of interactions in the scalar matter making up these stars. In particular, we show the pivotal role the scalar phase and vortex structure play during the late inspiral, merger, and post-merger oscillations of a binary boson star, as well as their impact on the properties of the merger remnant. To that end, we construct constraint satisfying binary boson star initial data and numerically evolve the nonlinear set of Einstein-Klein-Gordon equations. We demonstrate that the scalar interactions can significantly affect the inspiral gravitational wave amplitude and phase, and the length of a potential hypermassive phase shortly after merger. If a black hole is formed after merger, we find its spin angular momentum to be consistent with similar binary black hole and binary neutron star merger remnants. Furthermore, we formulate a mapping that approximately predicts the remnant properties of any given binary boson star merger. Guided by this mapping, we use numerical evolutions to explicitly demonstrate, for the first time, that rotating boson stars can form as remnants from the merger of two non-spinning boson stars. We characterize this new formation mechanism and discuss its robustness. Finally, we comment on the implications for rotating Proca stars.