Diluted Axion Star Collisions with Neutron Stars
  Diluted axion star, a self-gravitating object with the quantum pressure
balancing gravity, has been predicted in many models with a QCD axion or
axion-like particle. It can be formed in the early universe and composes a
sizable fraction of dark matter. One could detect the transient radio signals
when it passes by a magnetar with the axion particle converted into photon in
the magnetic field. Using both numerical and semi-analytic approaches, we
simulate the axion star's dynamic evolution and estimate the fraction of axion
particles that can have a resonance conversion during such a collision event.
We have found that both self-gravity and quantum pressure are not important
after the diluted axion star enters the Roche radius. A free-fall approximate
can capture individual particle trajectories very well. With some optimistic
cosmological and astrophysical assumptions, the QCD axion parameter space can
be probed from detecting such a collision event by radio telescopes.
