# Gravitational Lensing Signatures of Axion Dark Matter Minihalos in Highly Magnified Stars

## TL;DR

This paper proposes a novel method to detect axion dark matter minihalos through their subtle gravitational lensing effects on highly magnified stars near caustics in galaxy clusters, revealing potential signatures of axion dark matter.

## Contribution

It introduces a new observational approach to identify axion minihalos via photometric monitoring of caustic transiting stars, focusing on their collective lensing signatures in galaxy clusters.

## Key findings

- Axion minihalos produce surface density fluctuations detectable through microlensing light curve irregularities.
- Minihalos with masses around 10^{-15} to 10^{-8} solar masses can cause measurable lensing effects.
- Most minihalos are likely stable over cosmic timescales, avoiding disruption.

## Abstract

Axions are a viable candidate for Cold Dark Matter (CDM) which should generically form minihalos of sub-planetary masses from white-noise isocurvature density fluctuations if the Peccei-Quinn phase transition occurs after inflation. Despite being denser than the larger halos formed out of adiabatic fluctuations from inflation, axion minihalos have surface densities much smaller than the critical value required for gravitational lensing to produce multiple images or high magnification, and hence are practically undetectable as lenses in isolation. However, their lensing effect can be enhanced when superposed near critical curves of other lenses. We propose a method to detect them through photometric monitoring of recently discovered caustic transiting stars behind cluster lenses, under extreme magnification factors $\mu \gtrsim 10^3$--$10^4$ as the lensed stars cross microlensing caustics induced by intracluster stars. For masses of the first gravitationally collapsed minihalos in the range $\sim 10^{-15}$--$10^{-8}\,h^{-1}\,M_\odot$, we show that axion minihalos in galaxy clusters should collectively produce subtle surface density fluctuations of amplitude $\sim 10^{-4}$--$10^{-3}$ on projected length scales of $\sim 10$--$10^4\,$AU, which imprint irregularities in the microlensing light curves of caustic transiting stars. We estimate that, inside a cluster halo and over the age of the Universe, most of these minihalos are likely to avoid dynamic disruption by encounters with stars or other minihalos.