Surface conditions in accreting neutron stars

Abstract

The structure of the boundary layer where the accretion disk interacts with an accreting neutron star surface is studied using a one-zone approximation to observe the effects of the accretion on the thermal structure of the surface layers and on the progress of shell burning. The kinematic viscosity is parameterized by means of the critical Reynolds number, R/sub cr/. It is found that for higher accretion rate M/M/sub Edd/>>1/R/sub cr/ (M/sub Edd/ being the critical accretion rate corresponding to the Eddington luminosity), an optically thick boundary layer is formed, where radiation pressure is dominant. As long as it remains optically thick, the observable properties of the boundary layer are rather insensitive to the assumption of viscosity and are determined solely by the accretion rate. The internal structure, however, depends strongly on assumed magnitude of the viscosity as well as on the accretion rate; the pressure and temperature increase rapidly with R/sub cr/ and also with M. For large R/sub cr/, therefore, the boundary layer lies well inside the stellar photosphere, where the dissipation of kinetic energy plays a critical role in heating the envelope and in leading to the ignition of shell flashes. Results are discussed on the possible modificationsmore » of the X-ray burst model from that constructed under spherical symmetry and on their relevance to X-ray observations of low-mass binary systems.« less