On the mechanism of decelerating the gas infalling onto magnetized neutron stars

The problem of the deceleration of ionized hydrogen plasma is examined for the case in which the gas is infalling onto the surface of a strongly magnetized neutron star. The dynamic and the thermal structure of the lower part of the accretion column is obtained by solving the time-independent hydrodynamic equations together with the radiation moment equations. Depending on the accretion rate per unit area the deceleration of the infalling protons is achieved either by Coulomb friction with the ambient plasma at the lowest part of the column, or in a radiative collisional shock just above the surface. The possibility of collisionless, adiabatic shock is analyzed carefully. Such a shock front cannot exist above magnetized neutron stars, unless the accretion rate per unit area is smaller than approximately 10,000 g/sq cm s.