Axionic Boson Stars in Magnetized Conducting Media
Axions are possible candidates of dark matter in the present Universe. They have been argued to form axionic boson stars with small masses $10^{-14}M_{\odot}\sim 10^{-11}M_{\odot}$. Since they possess oscillating electric fields in a magnetic field, they dissipate their energies in magnetized conducting media such as white dwarfs or neutron stars. At the same time the oscillating electric fields generate a monochromatic radiation with energy equal to mass of the axion. We argue that the effect of the energy dissipation can be seen in the old white dwarfs. In particular, We show that colliding with sufficiently cooled white dwarfs, plausible candidates of MACHO, the axionic boson stars dissipate their energies in the dwarfs and heat up the dwarfs. Consequently the white dwarfs in the halo can emit detectable amount of thermal radiations with the collision. On the other hand, the monochromatic radiations can be seen only during the collision; a period of the dwarf passing the axionic boson star. Assuming that MACHO are dark white dwarfs, we show that there is a threshold in luminosity function of the white dwarfs below which the number of the white dwarfs in the halo increase discontinuously. The threshold in the luminosity function is expected to be located around $10^{-5.5}L_{\odot}\sim 10^{-7}L_{\odot}$. Its precise value is determined by the mass of the axionic boson stars dominant in the halo.