Axion boson stars

TL;DR

This paper explores new types of axion boson stars with self-interactions, revealing stable, compact configurations that could be detectable via gravitational waves, offering insights into ultralight axion-like particles.

Contribution

It introduces novel solitonic solutions in Einstein-Klein-Gordon theory with a periodic scalar potential, highlighting stability and compactness features not seen in standard boson stars.

Findings

New stability branches at high density

Existence of very compact, radially stable boson stars

Potential for gravitational-wave detection of these objects

Abstract

We study novel solitonic solutions to Einstein-Klein-Gordon theory in the presence of a periodic scalar potential arising in models of axion-like particles. The potential depends on two parameters: the mass of the scalar field $m_a$ and the decay constant $f_a$; the standard case of the QCD axion is recovered when $m_a\propto1/f_a$. When $f_a\to\infty$ the solutions reduce to the standard case of "mini" boson stars supported by a massive free scalar field. As the energy scale $f_a$ of the scalar self-interactions decreases we unveil several novel features of the solution: new stability branches emerge at high density, giving rise to very compact, radially stable, boson stars. Some of the most compact configurations acquire a photon sphere. When $f_a$ is at the GUT scale, a boson star made of QCD axions can have a mass up to ten solar masses and would be more compact than a neutron star. Gravitational-wave searches for these exotic compact objects might provide indirect evidence for ultralight axion-like particles in a region not excluded by the black-hole superradiant instability.