HARD X-RAY-SPECTRA FROM GAP ACCRETION ONTO NEUTRON-STARS

Weakly magnetized neutron stars undergoing disk accretion are found to emit copious hard X-rays if the marginally stable orbit lies outside the stellar surface. In this general relativistic picture (impossible in Newtonian gravity) the accretion disk terminates well outside the stellar surface. Accreting matter crosses the "gap" between the disk and the star in free fall; it then strikes the neutron star surface at a shallow angle, creating a hot equatorial accretion belt with a temperature inversion. For accretion rates that are not too high, the fluid flowing through the gap is optically thin to X-rays escaping from the equatorial belt. The structure of the surface boundary layer in such stars may then be largely decoupled from that of the disk. We exhibit a typical emergent spectrum, which was obtained in a self-consistent manner by a one-plus-one-dimensional radiative transfer and hydro code; power-law emission extends up to approximately 200 keV.