Angular distribution of radiation from low-mass X-ray binaries

The angular distribution of the X-ray emission from low-mass binary systems has been investigated under the assumption that an optically thin, geometrically thick disk developes in the vicinity of accreting neutron stars. Through scattering in the extended inner disk, radiation from the boundary layer is shown to be collimated in the direction perpendicular to the disk plane, and radiation from the stellar surface is enhanced in the same direction as a result of the reflection of photons incident upon the disk surface. The results suggest mass-radius relations for the burst sources EXO 0748-676 and 4U/MXB 1636-53 in the 2.2-3.4 Schwarzschild radius range. Under these conditions, the persistent emission is dominated by the radiation originating from the boundary layer and is then reprocessed in the inner disk.