X Rays from Old Neutron Stars Heated by Axion Stars

We show that axionic boson stars collide with isolated old neutron stars with strong magnetic field ($>10^8$ Gauss) and causes the neutron stars to radiate X ray by heating them. Surface temperatures of such neutron stars becomes$10^5 K \sim 10^6 K$. We suggest that these are possible candidates for X ray sources observed in ROSAT Survey. We discuss a possible way of identifying such neutron stars. We also point out that the collision generates a burst of monochromatic radiations with frequency given by axion mass.