  Axions have been argued to form coherent axionic boson stars as well as
incoherent axion gas. These are ones of most plausible candidates of dark
matters. Since the axionic boson stars generate oscillating electric fields in
an external magnetic field, they induce oscillating currents in magnetized
conducting media, which result in emitting radiations. We show that colliding
with a magnetic white dwarf, an axionic boson star can emit a monochromatic
radiation with a frequency given by a mass of the axion.
