Black hole formation from axion stars

By T. Helfer, D. Marsh, K. Clough, M. Fairbairn, Eugene A. Lim, R. Becerril. 2016

141 citations recorded by the source corpus.

The classical equations of motion for an axion with potential V(ϕ)=ma2fa2 [1−cos (ϕ/fa)] possess quasi-stable, localized, oscillating solutions, which we refer to as ``axion stars''. We study, for the first time, collapse of axion stars numerically using the full non-linear Einstein equations of general relativity and the full non-perturbative cosine potential. We map regions on an ``axion star stability diagram", parameterized by the initial ADM mass, MADM, and axion decay constant, fa. We identify three regions of the parameter space: i) long-lived oscillating axion star solutions, with a base frequency, ma, modulated by self-interactions, ii) collapse to a BH and iii) complete dispersal due to gravitational cooling and interactions. We locate the boundaries of these three regions and an approximate ``triple point" (MTP,fTP) ∼ (2.4 Mpl2/ma,0.3 Mpl). For fa below the triple point BH formation proceeds during winding (in the complex U(1) picture) of the axion field near the dispersal phase. This could prevent astrophysical BH formation from axion stars with fa ≪ Mpl. For larger fa ≳ fTP, BH formation occurs through the stable branch and we estimate the mass ratio of the BH to the stable state at the phase boundary to be 𝒪(1) within numerical uncertainty. We discuss the observational relevance of our findings for axion stars as BH seeds, which are supermassive in the case of ultralight axions. For the QCD axion, the typical BH mass formed from axion star collapse is MBH ∼ 3.4 (fa/0.6 Mpl)1.2 M⊙.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/784589a00bbd5207e2e7cdf38a6e42a931f7335c

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Citations in the corpus

The landscape of QCD axion models (2020, background): “As the density of the star increases, self-interactions become more relevant and destabilise the equilibrium when the mass of the star approaches the critical value [350, 356, 357]”
New experimental approaches in the search for axion-like particles (2018, background): “For the collapse to lead to a blackhole, values of fa close to mP are needed, see [255] for a discussion and state of the art simulations.”