Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Ultra High Energy Cosmic Rays from Axion Stars   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, $\sim 10^{15} (B/10^{12} {G}) {eV}
{cm}^{-1}$, 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. $\sim 10^{10}$ G. If we assume that the axion mass is $\sim 10^{-9}$ eV,
we can explain huge energies of the gamma ray bursts $\sim 10^{54}$ erg as well
as the ultra high energies of the cosmic rays $\sim 10^{20}$ 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.

28
The categories of this paper are: alg-geom
2
The categories of this paper are: math
15
The categories of this paper are: nucl-ex
32
The categories of this paper are: atom-ph
25
The categories of this paper are: patt-sol
36
The categories of this paper are: cmp-lg
18
The categories of this paper are: econ
6
The categories of this paper are: astro-ph
29
The categories of this paper are: funct-an
37
The categories of this paper are: supr-con
27
The categories of this paper are: comp-gas
10
The categories of this paper are: q-bio
20
The categories of this paper are: acc-phys
24
The categories of this paper are: dg-ga
21
The categories of this paper are: adap-org
11
The categories of this paper are: hep-lat