http://arxiv.org/abs/1909.05515
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.
D. Guerra, C. Macedo and P. Pani
Fri, 13 Sep 19
45/70
Comments: 15 pages, 4 figures