Glowing Axion Stars at Radio Frequencies
Axion-like particles (ALPs) can be converted to photons either through the Primakoff
effect in an external magnetic field or through a parametric resonance if ALPs constitute part of the dark matter
and form a condensate in the form of so-called axion stars.
For a given ALP-photon coupling and stable ALP star configurations, the occurrence of parametric resonance requires a minimal ALP star mass - the photon critical mass.
ALP stars exceeding the photon critical mass can give rise to radio line emission if the ALP mass is in the micro-electron volt range.
Here, we entertain the possibilty that ALP stars grow beyond this photon critical mass due to accretion from the
surrounding galactic background of dark matter ALPs
and estimate the resulting possible radio line signals from the galactic halo which strongly
depends on the accretion model. Comparing with the observed radio background allows to put preliminary constraints
on ALP-photon coupling and accretion scenario.
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