Can QCD Axion Stars explain Subaru HSC microlensing?

TL;DR

This paper investigates whether QCD axion stars could explain the Subaru HSC microlensing event, analyzing various formation scenarios and concluding they are unlikely to be the cause.

Contribution

The study provides a comprehensive analysis of axion star formation scenarios and assesses their viability as microlensing candidates in the Subaru HSC data.

Findings

QCD axion stars are unlikely to cause the Subaru HSC microlensing event.

Multiple formation pathways for axion clumps are considered and evaluated.

The candidate event is probably not due to QCD axion dark matter structures.

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.