Radiation-dominated boundary layer between an accretion disc and the surface of a neutron star: theory and observations

Observations of low-mass X-ray binaries in our Galaxy and external galaxies have drawn attention to the accretion disc boundary layer where the accreting matter slows down from its Keplerian orbital velocity of about half of the speed of light to a neutron star's rotational velocity and in which it releases about half of its gravitational energy. Correspondingly, a hot spectral component appears in the emission of accreting neutron stars, which is absent in accreting black holes. We review different approaches to the problem of the radiation-dominated boundary layer. In particular, we consider the theory of a levitating spreading layer, which assumes that the accreting matter slows down while spreading over the neutron star surface.