Stability of rotating scalar boson stars with nonlinear interactions

TL;DR

This paper investigates the stability of rotating scalar boson stars with nonlinear interactions, revealing that certain nonlinearities can suppress instabilities, and explores the nonlinear evolution leading to fragmentation or black hole formation.

Contribution

It demonstrates how nonlinear scalar interactions can stabilize rotating boson stars against known instabilities and analyzes their nonlinear dynamical outcomes.

Findings

Nonlinear interactions can quench non-axisymmetric instabilities in boson stars.

Instability persists in m=1 boson stars in the non-relativistic limit without nonlinear effects.

Some rapidly rotating boson stars remain stable over many dynamical times.

Abstract

We study the stability of rotating scalar boson stars, comparing those made from a simple massive complex scalar (referred to as mini boson stars), to those with several different types of nonlinear interactions. To that end, we numerically evolve the nonlinear Einstein-Klein-Gordon equations in 3D, beginning with stationary boson star solutions. We show that the linear, non-axisymmetric instability found in mini boson stars with azimuthal number $m=1$ persists across the entire parameter space for these stars, though the timescale diverges in the Newtonian limit. Therefore, any boson star with $m=1$ that is sufficiently far into the non-relativistic regime, where the leading order mass term dominates, will be unstable, independent of the nonlinear scalar self-interactions. However, we do find regions of $m=1$ boson star parameter space where adding nonlinear interactions to the scalar potential quenches the non-axisymmetric instability, both on the non-relativistic, and the relativistic branches of solutions. We also consider select boson stars with $m=2$, finding instability in all cases. For the cases exhibiting instability, we follow the nonlinear development, finding a range of dynamics including fragmentation into multiple unbound non-rotating stars, and formation of binary black holes. Finally, we comment on the relationship between stability and criteria based on the rotating boson star's frequency in relation to that of a spherical boson star or the existence of a co-rotation point. The boson stars that we find not to exhibit instability when evolved for many dynamical times include rapidly rotating cases where the compactness is comparable to that of a black hole or neutron star.