Variability and Stability in Radiation‐Hydrodynamic Accretion Flows

In this paper we examine time-dependent and three-dimensional perturbationsof spherical accretion flow onto a neutron star close to its Eddington limit.Our treatment assumes a Schwarzschild geometry for the spacetime outside theneutron star and is fully general relativistic. At all the accretion ratesstudied, the response of the accretion flow to perturbations includes weaklydamped oscillatory modes. At sufficiently high luminosities --- but still wellbelow the Eddington limit --- the flows become unstable to asphericalperturbations. These unstable radiation hydrodynamic modes resemble the onsetof convection, and allow accretion to occur preferentially through more rapidlydescending columns of gas, while the radiation produced escapes throughneighboring columns in which the gas descends more slowly.Comment: 31 pages. LaTeX, requires AAS LaTeX macros, version 4.0. 5 figures in encapsulated postscipt format. Accepted by the Astrophysical Journa