VARIABILITY AND STABILITY IN RADIATION-HYDRODYNAMIC ACCRETION FLOWS

In this paper we examine time-dependent and three-dimensional perturba tions of spherical accretion flow onto a neutron star close to its Edd ington limit. Our treatment assumes a Schwarzschild geometry for the s pacetime outside the neutron star and is fully general relativistic. A t all the accretion rates studied the response of the accretion how to perturbations includes weakly damped oscillatory modes. At sufficient ly high luminosities-but still well below the Eddington limit-the flow s become unstable to aspherical perturbations. These unstable radiatio n hydrodynamic modes resemble the onset of convection and allow accret ion 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.