Ultra high energy cosmic rays from axion stars

Abstract

We propose a model in which ultra high energy cosmic rays are produced by collisions between neutron stars and axion stars. The acceleration of such a cosmic ray is made by the electric field, ∼10 15 (B/10 12 G) eV cm −1 , which is induced in an axion star by the strong magnetic field B>10 12 G of a neutron star. As we have shown previously, similar collisions generate gamma ray bursts when the magnetic field is much smaller, e.g. ∼10 10 G. If we assume that the axion mass is ∼10 −9 eV, we can explain huge energies of the gamma ray bursts ∼10 54 erg as well as the ultra high energies of the cosmic rays ∼10 20 eV. In addition, it turns out that these axion stars are plausible candidates for MACHOs. We point out a possibility of observing monochromatic radiations emitted from the axion stars.