# On the final fate of compact boson star mergers

## TL;DR

This paper investigates the outcomes of binary mergers of compact boson stars using numerical simulations, revealing that head-on collisions produce a boson star, while boson-antiboson mergers lead to annihilation, and orbiting mergers form transient rotating bars.

## Contribution

It introduces a detailed numerical study of boson star mergers with various parameters using a modified CCZ4 formalism, providing new insights into their final states.

## Key findings

- Head-on collisions result in a boson star.
- Boson-antiboson mergers lead to annihilation.
- Orbiting mergers form a rotating bar that relaxes to a non-rotating boson star.

## Abstract

Boson stars, self-gravitating objects made of a complex scalar field, have been proposed as simple models for very different scenarios, ranging from galaxy dark matter to black hole mimickers. Here we focus on a very compact type of boson stars to study binary mergers by varying different parameters, namely the phase shift, the direction of rotation and the angular momentum. Our aim is to investigate the properties of the object resulting from the merger in these different scenarios by means of numerical evolutions. These simulations, performed by using a modification of the covariant conformal Z4 (CCZ4) formalism of the Einstein Equations that does not require the algebraic enforcing of any constraint, indicate that the final state after a head-on collision of low mass boson stars is another boson star. However, almost complete annihilation of the stars occurs during the merger of a boson-antiboson pair. The merger of orbiting boson stars form a rotating bar that quickly relaxes to a non-rotating boson star.