Transient Radio Lines from Axion Miniclusters and Axion Stars

TL;DR

This paper models the transient radio signals produced when dense axion dark matter clumps, like miniclusters and axion stars, encounter neutron stars, providing a framework for future radio search strategies.

Contribution

It introduces the first comprehensive modeling pipeline for transient radio signals from axion clump-neutron star encounters, detailing signal characteristics and aiding detection efforts.

Findings

Strong, narrow radio lines with durations from seconds to months predicted.

Characterized the brightness, spectral width, and temporal evolution of signals.

Provided a foundation for dedicated radio searches for axion clumps.

Abstract

Gravitationally bound clumps of dark matter axions in the form of 'miniclusters' or even denser 'axion stars' can generate strong radio signals through axion-photon conversion when encountering highly magnetised neutron star magnetospheres. We systematically study encounters of axion clumps with neutron stars and characterise the axion infall, conversion and the subsequent propagation of the photons. We show that the high density and low escape velocity of the axion clumps lead to strong, narrow, and temporally characteristic transient radio lines with an expected duration varying from seconds to months. Our work comprises the first end-to-end modeling pipeline capable of characterizing the radio signal generated during these transient encounters, quantifying the typical brightness, anisotropy, spectral width, and temporal evolution of the radio flux. The methods developed here may prove essential in developing dedicated radio searches for transient radio lines arising from miniclusters and axion stars.