Radiations from oscillating axionic boson stars in an external magnetic field

We solve numerically a field equation of axions coupled with gravity and show solutions representing oscillating axionic boson stars with small mass ∼10−12M⊙ and large radius ∼108 cm. We present explicitly a relation between the mass and the radius of the boson stars with such a small mass. These axionic boson stars are shown to possess oscillating electric currents in an external magnetic field and to radiate electromagnetic fields with a frequency given by a mass of the axions. We have estimated roughly the luminosity of the radiations in a strong magnetic field of neutron stars and have found that the radiations are strong to be observed.