Gravitational Wave Emission from Collisions of Compact Scalar Solitons

TL;DR

This paper numerically studies gravitational waves from head-on collisions of scalar solitons, revealing three outcomes depending on their compactness, including stable oscillatons, black-hole formation, and pre-merger collapse, with implications for gravitational wave signals.

Contribution

It provides a detailed numerical analysis of gravitational wave emission from scalar soliton collisions, identifying distinct outcomes and potential observational signatures.

Findings

Three collision outcomes depending on compactness: stable oscillaton, black-hole merger, pre-merger collapse.

Maximum gravitational wave energy exceeds that of black-hole mergers at certain compactness levels.

Post-merger quasi-normal mode amplitudes can distinguish scalar mimics from black-hole mergers.

Abstract

We numerically investigate the gravitational waves generated by the head-on collision of equal-mass, self-gravitating, real scalar field solitons (oscillatons) as a function of their compactness $\mathcal{C}$. We show that there exist three different possible outcomes for such collisions: (1) an excited stable oscillaton for low $\mathcal{C}$, (2) a merger and formation of a black-hole for intermediate $\mathcal{C}$, and (3) a pre-merger collapse of both oscillatons into individual black-holes for large $\mathcal{C}$. For (1), the excited, aspherical oscillaton continues to emit gravitational waves. For (2), the total energy in gravitational waves emitted increases with compactness, and possesses a maximum which is greater than that from the merger of a pair of equivalent mass black-holes. The initial amplitudes of the quasi-normal modes in the post-merger ring-down in this case are larger than that of corresponding mass black-holes -- potentially a key observable to distinguish black-hole mergers with their scalar mimics. For (3), the gravitational wave output is indistinguishable from a similar mass, black-hole--black-hole merger.