Accretion disk boundary layers around neutron stars: X-ray production in low-mass X-ray binaries

The boundary layer where the accretion disk meets the star is expected to b e the dominant source of high-energy radiation in low-mass X-ray binaries w hich contain weakly magnetized accreting neutron stars. We present Newtonia n solutions for the structure of the boundary layer in such a system. We fi nd that the main portion of the boundary layer gas is hot (greater than or similar to 10(8) K), has low density, and is radially and vertically extend ed. It will emit a large luminosity in X-rays, mainly produced by Comptoniz ation of soft photons which pass through the hot gas. The gas is generally optically thick to scattering but optically thin to absorption. Energy is t ransported by viscosity from the rapidly rotating outer part of the boundar y layer to the slowly rotating inner part, and this has the important effec t 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 depende nce 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 a viscosit y parameter and the viscosity prescription. Radiation pressure is the domin ant source of pressure in the boundary layer. The radiation flux in the bou ndary layer is a substantial fraction of the Eddington limiting flux even f or luminosities well below (similar to0.01 times) the Eddington luminosity L-Edd for spherically symmetric accretion. At luminosities near L-Edd the b oundary layer expands radially and has a radial extent larger than 1 stella r radius. This radial expansion increases the surface area of the boundary layer and allows it to radiate a larger total luminosity. Based on the temp eratures and optical depths which characterize the boundary layer, we expec t that Comptonization will produce a power-law spectrum at low source lumin osities. At high luminosities the scattering optical depth is quite large, and Comptonization of low-frequency bremsstrahlung photons will produce a q uasi-Planckian spectrum in the dense region where most of the energy is rel eased. This spectrum will be altered by Comptonization as the radiation pro pagates through the lower density outer boundary layer.