  If dark matter is mainly composed of axions, the density distribution can be
nonuniformly distributed, being clumpy instead. By solving the
Einstein-Klein-Gordon system of a scalar field with the potential energy
density of an axionlike particle, we obtain the maximum mass of the
self-gravitating system made of axions, called axion stars. The collision of
axion stars with neutron stars may release the energy of axions due to the
conversion of axions into photons in the presence of the neutron star's
magnetic field. We estimate the energy release and show that it should be much
less than previous estimates.Future data from femtolensing should strongly
constrain this scenario.
