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Radiation Burst by Axion Star Collision with Star in the Andromeda Galaxy

Axion is a promising candidate of dark matter in the universe. A fraction of dark matter axion may forms axion star with radius $\sim 10^2$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 $\sim 1.6\times 10^{-3}\mbox{Jy} (10^{-12}M_{\odot}/M_a)^2(10^{-5}\mbox{eV}/m_a)^3(B/10^2\mbox{G})^2\sqrt{3\times10^3\mbox{K}/T}$ and the rate of the collision per hour $\sim 0.06/\mbox{hour}\,(10^{-12}M_{\odot}/M_a)$ in the galaxy, where $M_a$ ( $m_a$ ) denotes the mass of axion star ( axion ) and $T$ does temperature of the electrons. We assume the number $10^{11}$ of the stars with $B\sim 10^{2}$G and radius $\sim 3.5\times10^{5}$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 $m_p\simeq m_a$. The duration of the burst lasts for the period which it takes the axion star to pass the region with $m_p\simeq m_a$. It would be longer than $1$ second.