Accretion onto neutron stars under adiabatic shock conditions

Abstract

The accretion of gas onto a neutron star is examined for the case in which a strong adiabatic shock front forms above the stellar surface to decelerate the incident plasma stream. Steady-state spherically symmetric flow is considered, all magnetic fields are ignored, and rapid thermalization by plasma instabilities in the shock front is assumed. The dynamic and thermal structure of the emission zone between the surface and the shock front is determined, and the emergent radiation spectrum is calculated. In many cases an appreciable fraction (about one-quarter) of the total energy is emitted in the form of gamma-ray photons between 1 and 10 MeV. The possible relevance of the model calculations to observations of galactic X-ray sources and gamma-ray bursts is discussed.