Near-critical spherical accretion by neutron stars: General relativistic treatment

Time-independent spherical accretion by a neutron star is studied using general relativistic radiation hydrodynamics. Numerical integrations of the flow equations are presented. These show that when the luminosity is sufficiently close to (but below) the Eddington limit, the flow velocity increases with decreasing radius far from the neutron star, reaches a maximum at an intermediate radius, and decreases at small radii. A large fraction of the binding energy of the flow is transferred to the radiation through scattering before the flow strikes the surface of the neutron star. Following Miller's treatment of accretion at luminosities near the Eddington limit (which neglected general relativistic effects), we derive analytic approximations for the decelerating phase of the flow's velocity profile. The dependence of the solutions on the variable Eddington factor prescription chosen to close the radiation moment equations is also examined.