Abstract
A non-negligible fraction of the QCD axion dark matter may form gravitationally bound Bose Einstein condensates, which are commonly known as axion stars or axion clumps. Such astrophysical objects have been recently proposed as the cause for the single candidate event reported by Subaru Hyper Suprime-Cam (HSC) microlensing search in the Andromeda galaxy. Depending on the breaking scale of the Peccei-Quinn symmetry and the details of the dark matter scenario, QCD axion clumps may form via gravitational condensation during radiation domination, in the dense core of axion miniclusters, or within axion minihalos around primordial black holes. We analyze all these scenarios and conclude that the microlensing candidate detected by the Subaru HSC survey is likely not caused by QCD axion stars.
Highlights
The current leading particle dark matter (DM) candidate is the QCD axion, being strongly motivated by shortcomings in the standard model of particle physics [1–3] and unification ideas in the frame of string theory [4–6]
Depending on the breaking scale of the Peccei-Quinn symmetry and the details of the dark matter scenario, QCD axion clumps may form via gravitational condensation during radiation domination, in the dense core of axion miniclusters, or within axion minihalos around primordial black holes
A major role in such setups is played by the so-called axion clumps or axion stars, which correspond to nonrelativistic Bose Einstein condensates (BECs) of axions
Summary
The current leading particle dark matter (DM) candidate is the QCD axion, being strongly motivated by shortcomings in the standard model of particle physics [1–3] and unification ideas in the frame of string theory [4–6]. Niikura and collaborators [29] report a single candidate for microlensing of stars (Ds ∼ 770 kpc) in the Andromeda galaxy (M31) after a dense-cadence, 7-hr-long observation of M31 with the Subaru Hyper Suprime-Cam (HSC) They propose primordial black holes in the M31 and Milky Way (MW) galactic halos as the cause for such an event constraining the PBH abundance in the mass range ∼1⁄210−10–10−5 M⊙ and excluding a PBH fraction of dark matter fPBH ≳ OðfewÞ × 0.01 [30]. We study such a claim analyzing the different known formation mechanisms of QCD axion stars We conclude that these compact objects, probably, would not cause the microlensing candidate detected by the Subaru HSC observation
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