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

Shvartsman first suggested that matter accreting onto a neutron star w ould produce observable gamma-ray spectral features, thus providing a measure of the gravitational redshift of the neutron star. We reconsid er the prevalent gamma-ray production channel: emission of 2.2 MeV pho tons from neutron-proton recombination. Nuclear spallation of a large fraction of the accreting helium (and heavier elements) liberates neut rons throughout the upper atmosphere. The eventual recombination of th e neutrons with atmospheric protons produces 2.2 MeV photons throughou t the atmosphere, a fraction of which escape unscattered. We have calc ulated the 2.2 MeV gamma-ray line flux and Compton-scattered continuum from this process. The flux of gravitationally redshifted 2.2 MeV pho tons from the brightest accreting X-ray source Scorpius X-1 is F2.2 al most-equal-to 10(-6)gamma cm-2 s-1, a factor of 20 below the sensitivi ties of the Compton Gamma-Ray Observatory. The next generation of gamm a-ray telescopes might observe this gravitationally redshifted spectra l feature if Scorpius X-1 is accreting matter rich in helium. The dest ruction of accreted He-4 leads to appreciable abundances of lighter el ements (D, H-3, He-3) in the upper atmosphere, which we determine. The destroyed He-4 is reformed through fusion reactions a few scale heigh ts beneath the photosphere, and thus presents no difficulties for X-ra y bursts. Our detailed discussion of the He-4-initiated nuclear spalla tion cascades and neutron diffusion effects may also be relevant to so lar flares, where the same nuclear processes occur.