Spread of Matter Over a Neutron-Star Surface During Disk Accretion

Disk accretion onto a slowly rotating neutron star with a weak magnetic field $H < 3\times 10^8$ gauss is considered in a wide range of luminosities $1/100 < L/L_{edd} < 1,$ where $L_{edd}$ is the Eddington luminosity. We construct a theory for the deceleration of rotation and the spread of matter over the stellar surface in the shallow-water approximation. The rotation slows down due to friction against the dense underlying layers. The deceleration of Keplerian rotation and the energy release take place on the stellar surface in a latitudinal belt whose meridional width rises with increasing $L.$ The combined effect of centrifugal force and radiation pressure gives rise to two latitudinal rings of enhanced brightness which are symmetric around the equator in the upper and lower hemispheres. They lie near the edges of differentially rotating and radiating upper and lower belts. The bright rings shift from the equatorial zone to higher latitudes when the luminosity $L$ rises. The ring zones are characterized by a minimum surface density and, accordingly, by a maximum meridional spread velocity. At a low accretion rate and luminosity, the released energy is removed through the comptonization of low-frequency photons.