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.
