HELIUM DESTRUCTION AND GAMMA-RAY LINE EMISSION IN ACCRETING NEUTRON-STARS

Shvartsman first suggested that matter accreting onto a neutron star would produce observable gamma-ray spectral features, thus providing a measure of the gravitational redshift of the neutron star. We reconsider the prevalent gamma-ray production channel: emission of 2.2 MeV photons from neutron-proton recombination. Nuclear spallation of a large fraction of the accreting helium (and heavier elements) liberates neutrons throughout the upper atmosphere. The eventual recombination of the neutrons with atmospheric protons produces 2.2 MeV photons throughout the atmosphere, a fraction of which escape unscattered. We have calculated the 2.2 MeV gamma-ray line flux and Compton-scattered continuum from this process. The flux of gravitationally redshifted 2.2 MeV photons from the brightest accreting X-ray source Scorpius X-1 is F2.2 almost-equal-to 10(-6)gamma cm-2 s-1, a factor of 20 below the sensitivities of the Compton Gamma-Ray Observatory. The next generation of gamma-ray telescopes might observe this gravitationally redshifted spectral feature if Scorpius X-1 is accreting matter rich in helium. The destruction of accreted He-4 leads to appreciable abundances of lighter elements (D, H-3, He-3) in the upper atmosphere, which we determine. The destroyed He-4 is reformed through fusion reactions a few scale heights beneath the photosphere, and thus presents no difficulties for X-ray bursts. Our detailed discussion of the He-4-initiated nuclear spallation cascades and neutron diffusion effects may also be relevant to solar flares, where the same nuclear processes occur.