Variability and Stability in Radiation Hydrodynamic Accretion Flows

In this paper we examine time-dependent and three-dimensional perturbations of spherical accretion flow onto a neutron star close to its Eddington limit. Our treatment assumes a Schwarzschild geometry for the spacetime outside the neutron star and is fully general relativistic. At all the accretion rates studied, the response of the accretion flow to perturbations includes weakly damped oscillatory modes. At sufficiently high luminosities --- but still well below the Eddington limit --- the flows become unstable to aspherical perturbations. These unstable radiation hydrodynamic modes resemble the onset of convection, and allow accretion to occur preferentially through more rapidly descending columns of gas, while the radiation produced escapes through neighboring columns in which the gas descends more slowly.