Radio signatures from encounters between Neutron Stars and QCD-Axion
  Minihalos around Primordial Black Holes
  Probing the QCD axion dark matter (DM) hypothesis is extremely challenging as
the axion interacts very weakly with Standard Model particles. We propose a new
avenue to test the QCD axion DM via transient radio signatures coming from
encounters between neutron stars (NSs) and axion minihalos around primordial
black holes (PBHs). We consider a general QCD axion scenario in which the PQ
symmetry breaking occurs before (or during) inflation coexisting with a small
fraction of DM in the form of PBHs. The PBHs will unavoidably acquire around
them axion minihalos with the typical length scale of parsecs. The axion
density in the minihalos may be much higher than the local DM density, and the
presence of these compact objects in the Milky Way today provides a novel
chance for testing the axion DM hypothesis. We study the evolution of the
minihalo mass distribution in the Galaxy accounting for tidal forces and
estimate the encounter rate between NSs and the dressed PBHs. We find that the
encounters give rise to transient line-like emission of radio frequency photons
produced by the resonant axion-photon conversion in the NS magnetosphere and
the characteristic signal could be detectable with the sensitivity of current
and prospective radio telescopes. It would be important to investigate in
detail search strategies for such signals which would provide a novel pathway
for QCD axion detection.
