Can QCD axion stars explain Subaru HSC microlensing?

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.

Automated summary

This paper discusses the possibility of gravitationally bound Bose Einstein condensates of QCD axion dark matter in the universe, as well as their impact on the single candidate event in the Andromeda galaxy from the Subaru Hyper Suprime-Cam microlensing search. The study concludes that the microlensing candidate detected by the Subaru HSC survey is likely not caused by QCD axion stars.