Generic initial data for binary boson stars

TL;DR

This paper develops a method to generate initial data for binary boson star simulations that satisfy Einstein's constraints, enabling realistic modeling of their gravitational wave signals.

Contribution

It introduces a new approach using the conformal thin-sandwich formulation with superposed solutions, reducing eccentricity and allowing diverse binary configurations including high spins.

Findings

Constructed constraint-satisfying initial data for binary boson stars.

Achieved low-eccentricity initial conditions (~10^{-3}).

Produced first gravitational waveforms from binary boson star mergers.

Abstract

Binary boson stars can be used to model the nonlinear dynamics and gravitational wave signals of merging ultracompact, but horizonless, objects. However, doing so requires initial data satisfying the Hamiltonian and momentum constraints of the Einstein equations, something that has not yet been addressed. In this work, we construct constraint-satisfying initial data for a variety of binary boson star configurations. We do this using the conformal thin-sandwich formulation of the constraint equations, together with a specific choice for the matter terms appropriate for scalar fields. The free data is chosen based upon a superposition of isolated boson star solutions, but with several modifications designed to suppress the spurious oscillations in the stars that such an approach can lead to. We show that the standard approach to reducing orbital eccentricity can be applied to construct quasi-circular binary boson star initial data, reducing the eccentricity of selected binaries to the $\sim 10^{-3}$ level. Using these methods, we construct initial data for quasi-circular binaries with different mass-ratios and spins, including a configuration where the spin is misaligned with the orbital angular momentum, and where the dimensionless spins of the boson stars exceeds the Kerr bound. We evolve these to produce the first such inspiral-merger-ringdown gravitational waveforms for constraint-satisfying binary boson stars. Finally, we comment on how equilibrium equations for the scalar matter could be used to improve the construction of binary initial data, analogous to the approach used for quasi-equilibrium binary neutron stars.