High‐Energy Gamma Rays from Old, Accreting Neutron Stars

We consider a magnetized neutron star with accretion from a companion star ora gas cloud around it, as a possible source of gamma rays with energy between$100$ $MeV$ and $10^{14}-10^{16}~eV$. The flow of the accreting plasma isterminated by a shock at the Alfv\'en surface. Such a shock is the site for theacceleration of particles up to energies of $\sim 10^{15}-10^{17}~eV$; gammaphotons are produced in the inelastic $pp$ collisions between shock-acceleratedparticles and accreting matter. The model is applied to old neutron stars bothisolated or in binary systems. The gamma ray flux above $100~MeV$ is not easilydetectable, but we propose that gamma rays with very high energy could be usedby Cherenkov experiments as a possible signature of isolated old neutron starsin dense clouds in our galaxy.Comment: AAS LATEX file; Accepted for publication in The Astrophysical Journa