Radiation burst by axion star collision with star in the Andromeda Galaxy

Abstract

Axion is a promising candidate of dark matter in the universe. A fraction of dark matter axion may form axion star with radius ∼102 km. We show that the axion star emits radiation burst by the collision with K and M types main sequence star in the Andromeda Galaxy. The emission arises in the atmosphere of the star, in which electrons coherently oscillate due to oscillating electric field of the axion star. The electric field is produced under magnetic field B of the star. We estimate the flux density of the radiation ∼1.6×10−3Jy(10−12M⊙/Ma)2(10−5eV/ma)3(B/102G)23×103K/T and the rate of the collision per hour ∼0.06/hour(10−12M⊙/Ma) in the galaxy, where Ma (ma) denotes the mass of axion star (axion) and T does temperature of the electrons. We assume the number 1011 of the stars with B∼102 G and radius ∼3.5×105 km in the galaxy. We also assume that a half of the dark matter is composed of axion star. We show that the emission of the radiation burst only arises in the atmosphere in which the plasma frequency mp≃ma. The duration of the burst lasts for the period which it takes the axion star to pass the region with mp≃ma. It would be longer than 1 second.