Accretion Disk Boundary Layers around Neutron Stars: X‐Ray Production in Low‐Mass X‐Ray Binaries

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The boundary layer where the accretion disk meets the star is expected to be the dominant source of high-energy radiation in low-mass X-ray binaries which contain weakly magnetized accreting neutron stars. We present Newtonian solutions for the structure of the boundary layer in such a system. We find that the main portion of the boundary layer gas is hot (108 K), has low density, and is radially and vertically extended. It will emit a large luminosity in X-rays, mainly produced by Comptonization of soft photons which pass through the hot gas. The gas is generally optically thick to scattering but optically thin to absorption. Energy is transported by viscosity from the rapidly rotating outer part of the boundary layer to the slowly rotating inner part, and this has the important effect of concentrating the energy dissipation in the dense, optically thick (to Thomson scattering) zone close to the stellar surface. Advection of energy also plays an important role in the energy balance. We explore the dependence of the boundary layer structure on the mass accretion rate and rotation rate of the star. We also examine the effects of changes in the ? viscosity parameter and the viscosity prescription. Radiation pressure is the dominant source of pressure in the boundary layer. The radiation flux in the boundary layer is a substantial fraction of the Eddington limiting flux even for luminosities well below (~0.01 times) the Eddington luminosity LEdd for spherically symmetric accretion. At luminosities near LEdd, the boundary layer expands radially and has a radial extent larger than 1 stellar radius. This radial expansion increases the surface area of the boundary layer and allows it to radiate a larger total luminosity. Based on the temperatures and optical depths which characterize the boundary layer, we expect that Comptonization will produce a power-law spectrum at low source luminosities. At high luminosities the scattering optical depth is quite large, and Comptonization of low-frequency bremsstrahlung photons will produce a quasi-Planckian spectrum in the dense region where most of the energy is released. This spectrum will be altered by Comptonization as the radiation propagates through the lower density outer boundary layer.