Optical Lensing by Axion Stars: Observational Prospects with Radio Astrometry

TL;DR

This paper proposes using radio astrometry to detect axion clumps acting as optical lenses, exploiting their photon coupling to distinguish them from gravitational lensing, and assesses SKA's sensitivity to these effects.

Contribution

It introduces a novel observational method using radio astrometry to detect axion clumps via their optical lensing effects, expanding the search for dark matter candidates.

Findings

SKA can detect lensing effects from axion clumps in currently unconstrained mass ranges.

Axion clumps cause distinct optical lensing, different from gravitational lensing.

Radio astrometry offers a new way to probe axion dark matter in the mass range $[10^{-14}, 10^{-11}] M_\u2212 0$.

Abstract

Axions and axion-like particles (ALPs) are some of the best-motivated dark matter (DM) candidates. Under certain circumstances, large axion fluctuations in the early universe can collapse to form dense configurations called axion clumps. The densest axion clumps are metastable states known as oscillons. In this paper we propose a new class of observables that exploit the axion's coupling to photons. As a result of this coupling an axion clump acts like an inhomogeneous refractive optical medium -- a lens -- that causes anomalous dispersion of incident electromagnetic waves. The dispersion of electromagnetic waves by axion clumps clearly distinguishes this lensing effect from gravitational lensing. Axion clumps passing in front of background radio sources act as lenses and lead to apparent positional shifts that can potentially be discovered by high-precision radio astrometry missions with the forthcoming Square Kilometer Array (SKA). We discuss the sensitivity of SKA to lensing effects in a variety of axion halo models. While gravitational microlensing surveys have placed strong constraints on the amount of DM that exists in the form of non-luminous astrophysical objects they have been unable to do so for objects in the mass range $[10^{-14}, 10^{-11}] M_\odot$. We find that, over a wide range of parameter space, SKA will be sensitive to optical lensing by oscillons in the mass range that is currently unconstrained by microlensing surveys.