A DISCRETE-TIME MODEL FOR INHOMOGENEOUS SPHERICAL ACCRETION ONTO NEUTRON-STARS

In this paper we present a phenomenological model for accreting neutron stars at high mass accretion rates. The optically thick infalling plasma and the outgoing radiation are described as two interfacing fluids in the gravitational field of the compact star. The strong interaction of the fluids causes the mass and radiation flow to be highly inhomogeneous. We assume that the plasma is trapped in large localized clumps arising through a Rayleigh-Taylor instability at the boundary between the fluids near the neutron star surface. The radiation created through the impact of the clumps on the surface is assumed to form bubbles of approximately the same size. The high-energy density of the bubbles is sufficiently large to push the matter above them out, creating channels in the plasma flow. The interaction between the two fluids can be described in time-discrete evolution equations of the clumps, bubbles, and channels. When we combine this with the "average" continuity equations for the mass, energy, and momentum for this highly inhomogeneous accretion scenario, we can calculate the luminosity of the neutron star as a function of the mass accretion rate. This calculation shows that for large mass accretion rates luminosities substantially exceeding the Eddington limit can be obtained.