Gravitational waves and kicks from the merger of unequal mass, highly compact boson stars

TL;DR

This paper investigates the merger dynamics and gravitational wave signatures of unequal-mass boson star binaries using numerical relativity, revealing unique features like large recoil velocities and potential distinguishability from black hole mergers.

Contribution

It presents the first detailed numerical study of unequal-mass boson star mergers with high mass ratios, highlighting new dynamical behaviors and gravitational wave characteristics.

Findings

Merger outcomes include nonspinning boson stars or spinning black holes.

Large recoil velocities up to 10^4 km/s are produced.

Distinct gravitational wave signals may allow differentiation from black hole mergers.

Abstract

Boson stars have attracted much attention in recent decades as simple, self-consistent models of compact objects and also as self-gravitating structures formed in some dark-matter scenarios. Direct detection of these hypothetical objects through electromagnetic signatures would be unlikely because their bosonic constituents are not expected to interact significantly with ordinary matter and radiation. However, binary boson stars might form and coalesce emitting a detectable gravitational wave signal which might distinguish them from ordinary compact object binaries containing black holes and neutron stars. We study the merger of two boson stars by numerically evolving the fully relativistic Einstein-Klein-Gordon equations for a complex scalar field with a solitonic potential that generates very compact boson stars. Owing to the steep mass-radius diagram, we can study the dynamics and gravitational radiation from unequal-mass binary boson stars with mass ratios up to $q\approx23$ without the difficulties encountered when evolving binary black holes with large mass ratios. Similar to the previously-studied equal-mass case, our numerical evolutions of the merger produce either a nonspinning boson star or a spinning black hole, depending on the initial masses and on the binary angular momentum. We do not find any evidence of synchronized scalar clouds forming around either the remnant spinning black hole or around the remnant boson stars. Interestingly, in contrast to the equal-mass case, one of the mechanisms to dissipate angular momentum is now asymmetric, and leads to large kick velocities (up to a few $10^4\,{\rm km/s}$) which could produce wandering remnant boson stars. We also compare the gravitational wave signals predicted from boson star binaries with those from black hole binaries, and comment on the detectability of the differences with ground interferometers.